As I’ve collected animus from various monsters across the spiral I ran into a few undead creatures that currently don’t give animus but should. Here is a list of the creatures I've run into:
The Phantom Devotee in the Ruined Alcazar, Khrysalis doesn’t give animus.
Cree Curdwright in Mausoleum, Khrysalis doesn’t give animus.
King Acampi Reefist in Cenote, Azteca doesn’t give animus. He doesn’t look like an undead but he is according to the Azteca storyline.
Lost Knight (boss) in Ruined Alcazar, Khrysalis does give animus, but he doesn’t appear in the monstrodome after you collect it so you can’t use the animus.
Lost Knight (minion to the boss) in Ruined Alcazar, Khrysalis doesn’t give animus.
Condemned Mander in House of Scales, Krokotopia doesn’t give animus.
Temple Guardian in Hollow Mountain, Shangri Ba doesn’t give animus.
Overlord Xerxes in Alkali Barrows, Mirage doesn’t give animus. This may or may not be intended. According to the storyline Overlord Xerxes is an undead king who was resurrected, but later in the story you find out that it was the Scorpion (not an undead) in disguise. I think you should be able to collect his animus though!
I hope this list helps wheedle out the bugs! In my opinion Monstrology is by far the best non-world update!