Actually it didn't even appear in the bestiary. For some reason it did not register.
#Trevor not unlocking
thick zinc
solemn flicker
were you playing in co op?
thick zinc
No. Single player. I just killed it again and it unlocked. I'm not sure why it did not register. On my first run in the Castlevania dlc on this file, i killed every boss initially available and made sure by checking the map.
I checked after every kill and no red skulls. It might have been a one off issue, but it for sure didn't register that first time as even a bestiary entry.
I can say Trevor should have been the first unlock on that run. (Correction, second unlock) There were 12 entries. Should have been 13. But i don't think that was it.
The new fusion of Shadow Pinion and Svarog statue may have killed it and could have prevented it from registering the first time. I really am not sure.