Crassus is one of the beasts found found within the Arena Rex world. The photos don’t really show it, but it’s a massive model. Amusingly, if you take it in your cohort, after you ‘kill’ it, it becomes a living hazard. Don’t poke the bear!
As a bonus, it also comes with this secondary model. Crassus has been in the pit for some time, evidently. Perhaps a second hazard?