Legend has it that the creature inhabited the area of Nerluc in Provence, France, and devastated the landscape far and wide. The Tarrasque was a sort of dragon with six short legs like a bear's, an ox-like body covered with a turtle shell, and a scaly tail that ended in a scorpion's sting. It had a lion's head.
The Tarrasque was said to have come from Galatia which was the home of the legendary Onachus, a scaly, bison-like beast which burned everything it touched. Some speculate that the story of the Onachus may be related to either that of the Unicorn or the Phoenix. The Tarrasque was the offspring of the Onachus and the Leviathan of biblical account.
The king of Nerluc had attacked the Tarrasque with knights and catapults to no avail. But Saint Martha found the beast and charmed it with hymns and prayers, and led back the tamed Tarasque to the city. The people, terrified by the monster, attacked it when it drew nigh. The monster offered no resistance and died there. Martha then preached to the people and converted many of them to Christianity. Sorry for what they had done to the tamed monster, the newly-Christianized townspeople changed the town's name to Tarascon.
The story of the Tarrasque is also very similar to the story of Beauty and the Beast and King Kong. The monster is charmed and weakened by a woman and then killed when brought back to civilization. A similar idea is found in the myths of Enkidu and the unicorn: both are calmed by sending them a woman.
"A dragon said to dwell in the Rhone River of central France. It is the child of the sea creature Leviathan and the monster Onachus. The Tarasque has the head of a lion and a sharp, pointed tail. It has a turtle-like shell covered with countless thorns on its back, and it breathes poison gas and leaves flaming droppings in its wake. It has a ferocious nature and will catch and eat human children and young women. While on a journey to preach the Gospel, The Christian saint Martha visited the village or Nerluc, where the Tarasque lived. When the beast attacked her, she did not waver, but Held out her crucifix and sprinkled holy water on it, and the Tarasque became docile. Drawn by this amazing sight, the villagers gathered and attacked the beast en masse and drove it away. The place where the Tarasque was defeated was named Tarascon. There, people celebrate the Tarasque Festival each year on the last Saturday in June. A woman dressed as Saint Martha and a float shaped like the Tarasque parade through town, commemorating the legendary event."
—Shin Megami Tensei IMAGINE Compendium
Tarasque could be contracted with in Shibuya's Celu Tower silver-level instance prior to the rebalance on the Japanese server in 2013. It appears as a field boss in Shinjuku Babel's docks area, a nightmare variant mob in Celu Tower's gold instance, and alongside Nidhoggr when he appears as boss in Shinagawa's Catacombs of the Templar Knight during the new moon.
Devil Summoner: Raidou Kuzunoha Vs. The Soulless Army
"A dragon-like monster said to have lived in the Rhone, near central France. It had six legs, and a large mouth with long fangs so as to easily swallow humans whole."
—Devil Summoner: Raidou Kuzunoha Vs. The Soulless Army Devil Chart
The Tarrasque blocks Raidou Kuzunoha the XIV from progressing farther underground in search of Kaya Daidouji. He demands liquor before he will allow anyone to pass. After being given wine, he decides to eat Raidou, which results in the second boss fight in the game. After being defeated, the Tarrasque requests more alcohol, in which Raidou gives him a pricey beer. Eventually, Raidou is able to ride the Tarrasque to the other side. On a later investigation, Raidou negotiates a pact with the demon that allows him to summon the Tarrasque whenever he needs to travel over water.
Devil Summoner: Raidou Kuzunoha vs. King Abaddon
A Mokoi makes a request to Raidou in the Case File It Came From the Sewer to find his old friend Tarrasque, who is lost. Raidou can find him in Tsukigata Village where three kids on picking on the small, weird turtle. Raidou must bribe them to make them leave him alone. Tarrasque reverts to his true size and thanks Raidou for his help, giving him a box that would grant him eternal life so long as it isn't opened, only to accidentally trip and open the box, forcing Raidou's HP and MAG to 1. He also gives the Flute of Rhione, which allows Raidou to summon him on the map when near water.