On herby island of you are unlucky and parked your raft in the cove, brontos sometimes get attacked by sharks and die to their large range can usually hit the raft or any tames you have in a water pen or if you are careless in the cove with no protection
I was tamping two lvl 40s for breeding, I got them both tamed and enabled breeding. I had forgot to look at the gender before i tamed, they were both girls