The problem with the Pathfinder hydra is, that it's rather easy to behead one now. In 3.5, you took at least 5 attacks of opportunity if you tried to sever one of its heads without the improved sunder feat. Now you only take one. With the change to cantrips, it's also much easier to stop the head from regrowing, at least if the group contains a wizard or sorcerer with acid splash.
I used a MM hydra in my beta game, a 5-headed one against four, rather underequipped 4th level characters. While they were shocked at first concerning it's fast healing 15, they defeated it in a few rounds without any casulties.
Without the extra attacks of opportunity, the hydra seems rather weak, especially the advanced versions.
Also, the hydra doesn't qualify for Combat Reflexes.
Concerning the Bonus Bestiary, if it's no longer planned to include its monsters in Bestiary 2, I might have to get the book instead of the printed out version I use at the moment, although it seems to be rather hard to get here in europe.