You'd probably want to make a custom body for the shape you're talking about. I could make one tomorrow but it's late where I live and I'm pretty tired. You might want to give the attacks [ATTACKFLAG_WITH] and [ATTACKFLAG_CANLATCH], and the creature will use any weapons it finds, and its attack will involve clinging to the target (like a squid latching onto someone's face). You'll probably also want to give them [AMPHIBIOUS] so they can breathe in air or water. I just noticed that they have no SPEED tag, you might want to add that. Normal humanoids have 900 speed, elves have 700, and a lower number is faster. And your creatures will never die of old age, but they also can't reproduce, so you may want to give them a MAXAGE tag and [CHILD:1]. And just for fun you could add a BEACH_FREQUENCY tag, so they wash up on the beach and massacre anyone nearby occasionally. And again, they'll mostly end up dead without [DAMBLOCK:25]. And add [LARGE_PREDATOR] and they'll kill anything smaller than them. [EVIL] makes them evil, but it also makes them not appear in good biomes. And if you're going to make them badass they need a MODVALUE to make killing them worth the effort. They'll really start destroying stuff if you give them [CAN_LEARN] so every time they kill something they get some experience towards being more effective at maiming your dwarves.
Well that's all I can think of at the moment, good night/morning/whatever time it is for you, and I hope your mod works out.