In the Dungeons & Dragons fantasy roleplaying game, thessalmonsters are a group of related creatures designed to resemble the hydra.
The thessalhydra first appeared in first edition in the original Monster Manual II (1983). The thessalmera appeared in Dragon #94 (February 1985).
The thessalmonster (including the thessalgorgon, the thessalhydra, the thessalmera, and the thessaltrice) appeared in second edition for the Forgotten Realms setting in the Monstrous Compendium Forgotten Realms Appendix (1989), and reprinted in the Monstrous Compendium Annual Volume Three (1996).
The thessalhydra appeared in third edition in Dungeon #134 (May 2006).
This family of hybrid monsters was created by Thessalar, a powerful lich of the World of Greyhawk campaign setting dwelling in the Wormcrawl Fissure.
The first thessalmonster was so genetically unstable that it could breed with other monsters to create new variants. When the original was killed, no new breeds of thessalmonsters could be made.
The thessalhydra is probably the first and most well-known of the thessalmonsters. It resembles a hydra for the most part, with a few differences. Instead of its eight heads emerging straight from the reptilian body, they form a ring around a large, circular mouth rimmed with jagged teeth. The tail ends in a set of pincers that can grab an opponent and force it into the large mouth. The main mouth can spit out a gob of acidic saliva once every day.
The thessalmera is a hybrid of the thessalhydra and chimera. It resembles the thessalhydra almost exactly, except that instead of the giant maw in the center of the snake heads, it has a leonine head. On the upper back above the snake heads is the head of a red dragon, which can of course breathe fire.
The thessalgorgon is a hybrid of the thessalhydra and gorgon. It looks mostly like the thessalhydra, though instead of the large mouth in the center of the creature, it has a head like that of a bull which can breathe out a cone of petrifying gas. Its entire body is covered in thick, metallic scales that are a coppery green in color.
The thessaltrice is an unlikely combination of thessalhydra and cockatrice. It looks just like a thessalhydra, except that the long necks are topped with heads like that of a cockatrice instead of snake heads. Each head is able to petrify a victim with a bite.
In the online television series Stranger Things, the characters play a game of Dungeons & Dragons featuring a thessalhydra.