Originally posted by Trakehner:

Just unlucky. I get lots of those. It's the stone golem trophies or wraith that seem the rarest.

5300 hours and I've had 2 of each that whole playtime.

I'm not sure I've gotten either of those, but I got my first sea serpent trophy the other day. I don't normally hunt those, so it's just a rare kill here and there, but one followed me into my home port, and was swimming the shallows, so I killed him with my sword. I got the meat, scales, and a trophy!