I'm not going to go through all of the information here on how it works, because Elijah DeathShard and his pals explained it so well over on this thread on Wizard 101 Central. We've really got to thank you, Mr. DeathShard, for collaborating with the entire community and putting together this excellent guide for everyone to use when making awesome hybrid pets!
There's just one thing that DeathShard might want to explain a little better in his guide. Here is what you see when you get down to Orthrus in the guide:
Alright, so it says "Orthrus with...". And then lists all the Level 48 Pets that have been tested yet. For example, here is says "Hydra-". Then whatever comes after that is what the description of the hatched pet is. Hydra + Orthrus = Rockythrus, in other terms.
Excellent guide! And the reason I haven't posted for the past few days is because I was away on a family retreat bit with my family. Since the day isn't close to over, I'm going to listen to the 16th Ravenwood Radio and then hopefully (Haley MoonCatcher beat me to the computer when we got home; she's fast learning the ways of Wizard 101 :P) get some "hybriding" time on the Test Realm, etc.
Keep on Casting,