The dimension gets created but there are no mobs (neither vanilla nor dinosaurs) in it.

CustomMobSpawner is a dependency of MoCreatures (see https://www.mocreatures.org/downloads ).

Are you sure you are on the right version? This has been fixed in version 0.6.

Yes:

~/forge-server-1.12.2$ ls mods/ | grep Prehistoric
Prehistoric+Eclipsev0.6-BETA.jar
~/forge-server-1.12.2$ ls mods/ | grep CustomMob
CustomMobSpawner-3.11.4.jar

I'll look into it, thanks for reporting the bug.

No spawns in the dimension when CustomMobSpawner is installed