Bipedal hyenas, lava bears, werecrocodiles and sea serpents? Never let it be said that fantasy writers have exhausted the realm of ideas!
As for any fantastic creatures/intelligent races... What springs to mind first is birds. If you've got a tropical environment, you'll most likely get tropical varieties of birds - and here, you can really go wild, considering the sheer diversity of real life tropical birds.
And then, after a quick trawl through Wikipedia on Zoogeography, I came up with Sea Turtles. If you've already got sea serpents, there's no reason why you can't have sea turtles as well. And considering that turtles have this whole thing where they live in the ocean and only come to land to lay their eggs, you've got a perfect set-up for a semi-migratory society.