Pet hatching was introduced in May 2010 and it created a way to produce rare hybrids of the two pets used in the hatching process. Hybrid pets result from hatching two adult pets and getting a result that is not the same as either of the two parents. This can be an entirely unique pet, only available through hatching a hybrid, such as the Archfiend, Grimzilla, Lifezilla and many others.
In the April KingsIsle Live event, Community Managers revealed that there are still 100 undiscovered pet hybrids, which is a lot considering pet hatching has been around for almost 5 years. During the show, Professor Greyrose and Dworgyn also confirmed the following pet mixing results:
Clockwork Golem + Black Spider = Spider Golem
Minotaur + Treant = Life Minotaur
Red Gobbler + Red Gobbler = Pet Rock
Also, in the May 2015 Newsletter, KingsIsle unveiled more combinations that can produce hybrids (remember: hybrids can be rare so if you don't get it the first time, don't give up):
Frostman + Seraph = ?
Black Cat + Christmas Elf = ?
Fire Elf + Grumpy Snowman = ?
Pale Maiden + Dragon = ?
Krok + Brown Spider = ?
Tough Troll + Brown Spider = ?
Some of these undiscovered hybrids were already reported on Wizard101 Central. If you find more combinations or discover new hybrids, feel free to share your findings there.
Note: The hybrids in the banner above do not reflect actual pets in the game.