If you save while there are some dead beast bodies lying around, they will play idle animation when you reload.
Couldn't replicate this bug with the instructions provided. I loaded into hollow and spawned Skargs, Bafu and Oculi, killing each of them with the Purge God Mode ability.
Saving the game and reloading that same save caused the beast corpses to disappear instead. Which is in itself an issue so I'll push this through with an adjusted title but I would welcome replication steps for the dead beasts playing idle animation on reload.
From what I understand, this is the ghost unit issue, and it occurs somewhat randomly. It is not specific to beasts; it may occur with any unit.
For further review in the future.