User blog comment:Kapadie/Is this real Subnautica concept art?/@comment-26794840-20150831140124

The small creature is definitely a Spine eel, check it out here Spine Eel Sketchfab. The big skull might belong to a Sea Dragon Leviathan, with its big teeth and multiple eye socket.

Maybe we will see it in the soon-to-come™ blood kelp zone biome.