Color, size, and in mammals it might be horns. The females hide in the bird kingdom. The females in predators can be the hunters but males usually dominate. Hunt elk and a cow generally leads the herd and a bull travels along keeping his harem together. I figure the collectors of the specimens got what they could and didnt worry about human idiocy from the future. JMO
I was just kidding to poke fun at the SJWs who come up with this stuff but it is logical that some specimens are easier to get and some are simply more interesting to put in a collection.