Supposedly there is only one tarrasque in existence, but there are a few published instances that contradict this. Obviously, as the DM you can decide for yourself if this is a unique creature in your campaign.
I will also get this out of the way here at the top of the article. I love the Tony DiTerlizzi depiction of the tarrasque (to the left). I hate the more recent depictions. So in my game, it doesn't have a freaking Gamera shell.
Full disclosure: I once threw two 6-armed tarrasques at my party when I was 13 years old. The creatures were slaughtered by our insanely over-powered heroes (I didn't know what I was doing, in my defense).
I had no idea that this creature was based on a real life mythological creature out of France. It is known as the Tarasque.
It was mentioned in a 4th edition article: "The iconic D&D beastie is loosely based on the realworld myth of the Tarasque, a turtle-shelled French dragon that was charmed by a saintly woman and was slain, like King Kong, due to its weakness for beauty."
I did a little research and learned that the Tarasque was a sort of dragon with a lion head, six bear legs, a turtle shell and a scorpion tail. Saint Martha (her brother was Lazarus - he died, and Jesus Christ cast raise dead to bring him back) tamed it and led it back to a town where the people killed it. The people felt bad about what they did, so they became Christians and re-named their town Tarascon.
AD&D 1st Edition
When it makes a bite attack, if it rolls an 18 or higher, it severs an extremity of the victim.
When you see it, if you're level 3 or less, you can do nothing but stare in awe (no save!). If you're under level 7, you flee in panic. Level 7+ people can save vs fleeing.
- It is immune to magic missiles, lightning bolts and cones of cold. There's a 1 in 6 chance these spells are reflected back at the caster!
- It regenerates one hit point per round and is only harmed by magic weapons.
- To slay the tarrasque: Drop it to -30 hit points, cast wish and wish for its' death.
- Supposedly there's around 100,000 gp in gems in its' belly. Dwarf blacksmiths can use portions of its' hide to make d4 +5 shields.
- The tarrasque sleeps for 5-20 months, then rampages for 1-2 weeks.
From what I can find, the tarrasque appeared in a few 1e adventures:
- H2 Mines of Bloodstone & H4 Throne of Bloodstone: It is hanging out in a temple of Orcus..? Part of the temple has 8 rooms that Orcus uses to test followers or kill prisoners or whatever. One room has a tarrasque. Apparently somehow when it is over, the room "resets" and the tarrasque is brought back to life. Then in H4, it is encountered on the 333rd layer of the Abyss - Orcus' domain. It lives in a cave or something. In both instances, it's literally just a monster placed in a dungeon.
- WL16 The World Of Krynn: This book has four Dragonlance adventures. One of them is called Dargaard Keep, where the adventurers attack Lord Soth's keep. In the final battle, Soth waves his hand, a door opens and... the tarrasque lurches forward to attack. Welp, OK then.
AD&D 2nd Edition
Its' THACO is a -5. It's been a long time, and I know THACO gives people headaches. Here's why. In the olden days a low Armor Class (example: plate mail gives you an AC of 2) was good and an AC of 10 meant you were wearing no armor at all. What this means is that if it rolls a 1 to hit, it hits an AC of -5 or in 5e terms, an AC of 21!
So basically the tarrasque has a +21 to hit in 5e terms.
There's this really weird note at the bottom: "Creatures with a minus THAC0 can only be hit on a 1." What the hell does that mean?
I googled it and found that Skip Williams answered this in a column on wizards.com:
"Lee asks: 'The 2nd edition Monstrous Manual entry for the tarrasque ends with the sentence: "Note: Creatures with a minus THAC0 can only be hit on a 1." This makes no sense, as a monster's THAC0 can't possibly modify what something else needs to roll to hit them. I always thought it should have read that the tarrasque can only miss an attack by rolling a "1". My anal-retentive friends, however, will only acquiesce to an official ruling on this subject. Can you help me out?'
Ah, the tarrasque--a suitable topic to end this set of questions. Lee, that is indeed a truly bizarre statement from the Monstrous Manual, which defies both mechanical and thematic logic."
"....THAC0, as defined by the 2nd edition PHB, was the number a character needed to roll in order to hit AC 0. The THAC0 number could then be used to calculate the number needed to hit any Armor Class. This definition ends with the somewhat dubious statement that "using THAC0 speeds the play of combat greatly."
OK, that really wasn't very helpful. It looks like someone just got a little mixed up writing that note and nobody caught it or knew what the hell it meant.
In the ecology section, it is mentioned that the "elemental princes of evil" may have created the tarrasque. Or it may be a "great, reptilian punisher sent by the gods to end the world".
The tarrasque appears in these 2e products:
- How the Mighty Are Fallen: The heroes need to get the tarrasque's pituitary gland and give it to a wizard. Yup!
- Practical Planetology: On the world of Falx, there's apparently many tarrasques in a docile state, eating rocks.
- The Apocalypse Stone: This adventure was designed to end your 2e campaign so you're fresh for 3e, which was about to come out at the time of the adventure's release. I really wish they had done this in 4e. As the apocalypse is going on, the tarrasque goes on a rampage and the heroes must stop it.
The tarrasque appears in the 3e monster manual. It's pretty much the same deal as in earlier editions. It's gullet can hold 32 medium-sized creatures! A severed limb grows back in d6 minutes.
Usually the best part of doing research like this is getting to crack open a Paizo issue of Dragon Magazine. The Ecology of the Tarrasque appeared in the final printed issue (#359), an issue I've barely glanced at. This issue is full of awesome stuff: Time-traveling dragons, a Demonomicon of Iggwilv entry, Greyhawk magic items. There's even an article that answers big questions like "Who is the Lady of Pain?" an "What are the Dark Powers of Ravenloft?". It also discusses out-of-game mysteries, like what happened to 2e Greyhawk designer Carl Sargent.
The ecology article starts off with a little bit of fiction about the "Monster Hunters Association". Members include people named Dreelix, Willowquisp, Spontayne and Buntleby. Oh, Ed Greenwood is writing this. Come on, now. Willowquisp?! Really, Ed? In my tarrasque article?
- Its' carapace is like melted diamond
- It has three stomachs, Swallowed stuff goes through all three. Stomach one punches, squeezes and claws at swallowed stuff. Stomach two is an acid cauldron and strips magic from magic items. Stomach three is a furnace. Waste is absorbed by the tarrasque.
- When it is time for hibernation, the tarrasque can earth-glide like a xorn, "swimming" through earth, leaving no tunnel or wake.
There's a wizards.com article expanding on the tarrasque. Mostly it's just three huge freaking stat blocks that some poor guy spent hours cooking up to create variant versions of the tarrasque. One version is for "modern d20" - basically it's Godzilla. Here are the other two:
The author suggests that perhaps the tarrasque grows five heads after tearing through and devouring a wizards' lab or tower. That seems like a really fun idea.
Spellwarped Woodling Tarrasque
It lives deep in the woods, resembling an ancient tree when dormant. It might live near a sinkhole of mage. Awesome!
The tarrasque appears in:
- The World's Largest Dungeon, an adventure that contains every single monster in D&D. The tarrasque is in a mine burrowing toward a pit that holds thousands of demons. That sounds pretty epic.
Looking at it now, though, it doesn't seem as bad as I recall. It has an aura which brings fliers close to it. It can make 2 attacks... sometimes. It can frenzy and attack many enemies.... when bloodied. But with 1420 hit points and resist 10 to all damage, it will take forever just to bloody it. The most damage a PC ever did in one round, if I remember correctly, was 180 points. And that's from an exceptional player using an action point.
The thing that sticks out the most to me here is the tarrasque's complete inability to deal with stun (you can't do anything on your turn when stunned) and daze (you only get one action on your turn). In epic tier, most classes can stun and daze quite a bit. Each round takes quite a while in real-life, and when your monster is stunned for a round or two, that's a half hour of real life time where the PCs are just teeing off on it. But with 1420 hit points, they're not making that much of a dent and it becomes a lame exercise of rolling hits and damage with people groaning "is it even bloodied yet?".
On top of that, if there was one monster in 4e that should have had threatening reach (which means it can make opportunity attacks from a few squares away), this is it! There were only a handful of monsters in all of 4th edition with threatening reach, and I never understood why. It was an incredibly potent feature that changed a fight completely. Just getting close enough to hit the monster with a melee attack is a challenge with threatening reach. And this is a monster that should be a challenge!
The monster got a big boost in Dragon Magazine #418. In one article, it is outright stated that The Tarrasque was created as a living weapon in the Dawn War. The primordials created it to spite the gods.
"Before their exile, the archomentals wrought one final act of spite. Combining their talents, they fashioned in secret an abomination of unquenchable hunger. Imix the Fire Lord infused the tarrasque with burning internal heat and murderous energy. Cryonax the Bringer of Endless Winter and Ogrémoch the Stone Tyrant gave it immortal endurance. Olhydra the Mistress of the Black Tide and Yan-C-Bin the Great Cloud inspired the creature’s inexorable wrath. There is evidence that at least two Elemental Princes did not survive this creation process—a force of acid and a creature of bone and blade—but their names have been lost."
The article points out that the tarrasque can swallow and destroy artifacts. It lists three that actually were destroyed: The Talisman of Ahrjez, the Ebon Band and the Fang of Baator.
The article also contains updated stats for the tarrasque. Less hit points and immunity to stun! Much better. They pumped up the damage and lets it make two attacks when it takes damage (once per round).
It can auto-save three times per day, and has advantage on saves vs. magic.
The spell reflection is handled well. Roll a d6. 1-5 means most ranged spells don't affect it. On a 6, the spell is reflected!
It makes 5 attacks. They do anywhere from 24-34 points of damage each.
Wow. When it swallows you, you are blinded and restrained and you take 56 damage at the start of each of the tarrasque's turns!
It has some pretty basic legendary actions it can use at the beginning or end of other creatures' turns.
There you go. Now wreak havoc upon your players!