Not sure actually. Hold on...
Ok. I started a miniature fort. Had a miner dig 7 z-levels straight down. Then I designated it as a pit and chucked animals in it.
The horse pushed the miner out of the way and the miner slammed into the wall. When this happened his right hand was broken. The horse died on impact.
Then I threw a yak bull in there. The miner had passed out from pain by now. The yak hit him and broke both his legs, crushed his torso and smashed his skull. The yak was only bruised by the impact.
I threw a dog down there. No effect. It missed the yak and smashed into the ground, breaking a leg and a lung.
I threw another dog down there. It struck the first dog, pushed him into the wall, and caused him to break his tail. The falling dog was uninjured.
I chucked in two cats too ... one did nothing. The other hit a dog without effect.
... and then I threw a steel anvil down there.
The anvil bruised the Yak's heart, crushed a cats head and upper spine, crushed the other cat's teeth and sent them flying, it crushed one dog's leg and then sent him into the wall where he sustained additional bruising, it also hit the other dog, causing mild injury, and sending him into the wall where he was bruised.
In conclusion I think that creatures now work as other items. What happens depends on weight... I am sure there has been a lot of research done into this but it's always fun to do more research! If you throw a cat on UristMcGetthekitty then he might not even notice it. If you throw an elephant on him, he might be crushed to death.