Hey yeah! Any one can create a monster as long as they submit here first, once approved, you can post it in the reference sheet!

And yes, sub-classes would be very helpful. Even if the beast your creating is a mountain cat of some kind, it would still be beneficial to the roleplay.

I also like the idea of a mermaid-like creature, but she has wings in this one. So this is what I had in mind: a Sea Harpy. They live along the coastlines in cliff faces where they build their nests, and they are notorious for attacking ships, eating the sailors, and hoarding loot. The Sea Harpies would have some magical capabilities, but nothing over-powering, although they would be dangerous in hand-to-hand combat.