The Hydra pet is an pet you get for finishing the level 48 quest, but I'm not sure what is the name of the quest. :?
The other schools pet are:
-Death: Wraith : Give one Wraith card -Myth: Othrus : Give one Othrus card -Life: Satyr : Give one Unicorn card -Fire: Hellephant : Give one Hellephant card -Ice: Collosus (I think) :Give one Collosus card (Not sure) -Storm: Stormzilla : Give one Stormzilla card (Not 100% sure about the card)
If you finish that quest you get a new card ;) Here are the cards:
-Death: Scarecrow -Myth: Othrus -Life: Rebirth -Fire: Fire Dragon -Ice: Ice Giant -Storm: Storm Lord