Is there a list somewhere of all the options? Particularly in the latters case, there seem to be some creatures that are supposed to be options, but don't have the level advancement or mastery stats listed.
By Monstrous Companion, do you mean Monstrous Cohort, from the Leadership feat? Here is a LIST of all options from Bestiary 1-4. THIS is all I could find on Monstrous Mounts, though the Monstrous Mount Mastery feat is from the Inner Sea Campaign Setting, so that's likely all that was ever made for it, as it's not from a Core Book (CRB, APG, ARG, ACG, Ultimate ____, ____ Adventures)
Yes, that is what I meant, my bad. That's about all I could find as well. Unfortunate.