Here's a nice list of things that are missable that you should try and get:

https://docs.google.com/document/d/1...2FI/edit?pli=1

And also two more missable summons:

Catoblepas - defeat him in a random battle in the forests on an island in the northwest corner, accessible only by submarine (World 2)
Carbuncle - defeat him in Exdeath's Castle (World 2)