Fuse two of your discovered creatures once they reach the appropriate level.

 
If you discover a creature that can be fused to create a hybrid, the initial hybrid form will also appear highlighted in purple in the "Discovered" section of your Collection. Even though you can see the hybrid, you won't be able to create it until your ingredient creatures are at a high enough level. 

 
Some creatures can be fused into multiple hybrids. Once you create a hybrid, other hybrid variations of that creature can appear in your Discovered creatures list. If the hybrid you've just made can itself be fused into a Superhybrid, it will also appear in your Discovered section.