After the post-game dungeon, I had two missing monsters. I found one via the summoning app (Hellbite). Now I have single monster left, and for the life of me I can't find where I could meet it. It's about halfway through the library list:

Gajigaji
Dragon Tamer
Dracomancer
???????
Megadoras
Mephisto
Yamata no Orochi

This would place it somewhere in the Abyssal cradle - the first part, before final boss. It's definitely not in the list of things I can summon, neither on land, nor in underwater. The map doesn't show any obvious boss arena I might have missed either.