Being iv worked on the same hydra model, I can give you a good review on this model because I know how hard the hydra is to mess with, with my 5 headed hydra still taking over 4 years.
Cons: the feet look really bad, I suggest deleting the toes to make them into hoofs, to better match other fel hounds legs.
I think you should give those back tentacle spines some love by either animating them, adding more of them, or removing them and replacing them with more spines similar to the tail end.
I think you broke the theme by making the spines higher up on the tail too different from the swarms of spines on the bottom of the tail, fix that by placing ones from the lower tail up there to replace them, those are the better looking spines any way.
I think the beards on their chins could use some work, perhaps lengthening them to hang lower so they can swing more
Pros: I love the way you made the tail, those spines look really really nice.
I like what you did with the heads, they fit very well. I noticed you did what I did and deleted a lot of bones, and reattached the entire mesh to the actually animated bones, deleting what over 35-40 bones total? that's a lot of space saved good job.
This is one of the most creative things iv seen a hydra turned into in all of google, worthy of my mur'loc map. I'll give it a 8/10 could be a 9 if you added some more custom spell animations, or perhaps fixed some of those things I mentioned.
good job on this one I very much admire it.