If you fallow this link:http://www.wizard101central.com/wiki/Pet:Hydraand scroll down this page you will find all pets that can be hatched with Hydra. If something happens and you can't open this link (sometimes happens) here is short version:
When I first started out I wanted Spritely and Spell Proof so I started by buying Starfish at the pet vendor in Celestia, I bought and rehatched them till I got Spell Proof and Spritely manifested. Then I did the Halloween quest and got Frankinbunnies and brought in Energizing Battery. Later I was able to get a hatch with a Brave Hound that had Unicorn and Fairy Friend and then got an Enchanted Armament pet drop helping a friend with the side boss in Olympus. Even later I got a random hatch Nightwalker that brought in Spell Defy. Now, about a yr later, I have a Nightwalker and an Enchanted Armament pet with Spell proof and defy, Fairy and Unicorn.
I did this on a lark with no time limit and many, many failed pets. My goal was a Spud Enchanted Armor pet which is yesterday's news really because healing talents are reduced mc's now. I have yet to try to hatch back in Spritely training past Epic is painful for me, lol.
I have collected some cute pets along the way and passed around a lot of Frankenbunnies and Enchanted Armor pets in many random hatches. I have also side tracked to put some time into a lil Siren pet for my Storm wizard.
Short cut = go to wizard101central.com, create an account and ask in the pet meet ups part for someone to hatch with you. There are many kind wizards there that will help you get the pet you want.