I will admit I haven't thought about this monster in years but something recently brought them to mind and now, after rereading the variations of this creature, I can see a lot of potential.

The thessalhydra was introduced in the 1e Monster Manual II. It was a massive creature that had a body similar to that of a normal hydra. The differences were significant- a huge mouth surrounded by the necks, a pair of tail pincers, acidic bites for the snake heads and massive maw, the ability to spit acid (doing 12d6 points of damage!), immunity to acids and acidic poisons and the ability to destroy items with the maw. Powerful, highly destructive and easily a top level predator. It could eat smaller black dragons without too much difficulty and easily dominate a swamp or wet cavern system.

In the second edition's Monstrous Compendium Annual Volume Three, it was turned into a family of monsters. The thessalmonster, mentioned as extinct, was ancestor to hybrids with hydras, chimeras, gorgons and (strangely) cockatrices. There is mention of unstable genetics allowing the hybrids and wizards creating the original species. All of them are hybrids of not the thessalmonster, but rather the thessalhydra. The thessalmera can't use it maw for some reason but replaces that with its lion head. It can also has a fire breath weapon in place of the acid spit. The thessalgorgon has a bull head, does retain the maw attack but loses the spit. It can turn creatures to stone four times per day with a breath weapon. The thessaltrice replaces the snake heads with cockatrice heads, each able to pretrify once per day, and can use its maw. Each of these monsters is of a different size and does varying amounts of damage. They also have obvious immunities.

In the third edition, they were turned into a template in Necromancer's Tome of Horrors (I have the revised version but I think they are in the original as well). I do not believe they have been converted to Pathfinder as of yet. The template mentions the wizards, unstable genetics and how the resulting monsters are hybrids of the hydra, not the original monster. It can be applied to any magical beast with 5 or more hit dice. It has three examples- the thessalhydra, thessalgorgon (with really bad art, replacing the bull head with a cobra) and the thessallisk (a hybrid with the basilisk). The resulting monster is an aberration and may have the pincers (used to immobilize prey) and will have either the maw or some other bite attack and immunity to acid as well as its original powers.

The template is interesting, and useful, but it doesn't go far enough into what the thessalmonster was and how it could have bred with everything from hydras to cockatrices. It hit me that this would be a perfect place to use a hemiclone. All thessalmonster variants are females that are clones of their mothers' side but still incorporate abilities and traits from their fathers*. They reproduce not by mating, but by consuming another monster. The strongest is used to fertilize the developing eggs and thus the mother may produce a different kind of thessalmonster. So a thessalhydra that eats a spider eater can lay eggs that will hatch into thessalspiders one year and then some time later eat a displacer beast and produce eggs that will hatch into thessaldisplacers. That is a very powerful effect and one that can drive thessalmonsters to find and consume as many other monsters as it can, both to find good fathers and to protect their offspring. This may seem like a good thing, but thessalmonsters are more powerful than the monsters they replace.

But why restrict them to magical beasts? Vermin, dragons, outsiders, monstrous humanoids and giants could all become thessalmonsters. Thessalmonster populations could expand far and wide, converting other monsters into more thessalmonsters. And then there could be thessalmonsters with more than one father. These mutants gain the traits, good and bad, of the most powerful monsters the mother consumes- she can decide which are added and which are excluded.

Thessalmonsters aren't quite the hazard that deepspawn are but they are close enough. If they start eating dragons and taking their traits, thessalmonsters could become a force that even dragons can not stop. If they can mix dragon traits with outsiders then the world may very well be brought to the brink.

* I have mentioned this before and it is found among a few species of real world fish. They breed with related species, taking half their genes from their fathers but then excluding those genes when making their own eggs.