What you think of when you hear the word Hydra: https://en.wikipedia.org/wiki/Hydra_(genus)
What the rest of us think when we hear the word Hydra: https://en.wikipedia.org/wiki/Lernaean_Hydra
(No, I don't speak for most of the community. But I'm sure if I ask some person on the street to describe a Hydra, this is what they'll think of.)
As for the suggestion itself, do each of those duplicate nine hydras spawn hydras when they're hurt, or just the first hydra? Also, why do you have to hit it 10 times with Fire Aspect in order to burn it? And where or how does this thing spawn? This seems like it has to be a boss mob, because I would lock the game on Peaceful if I had to fight this thing constantly just by being in a specific biome.