Elephants and unicorns...yea, not nearly as dangerous as they used to be.
Carp1 and hippos are still quite dangerous however. It seems that if dwarves hang out by the river too often they will take offense at the intrusion and will eventually attack.
Fish while technically not dangerous themselves cause dwarves that fight back to charge into the water and drown. (if they can beat the fish at 1/3 normal attack speed)
Hippos act like the elephants used to act back in 2D. Usually running away, but like fish, eventually taking offense at the intrusion and attacking, after which they usually still hang out around the inevitable sock and then slaughter dwarves one at a time, or chase one unfortunate soul across the map for a season and eventually getting lured into the fortress proper.
Both of these however rarely produce deaths once you know what you are doing and understand their behavior. Even if you aren't really paying attention, after the first death, even basic damage control skills can prevent additional deaths. Carp and hippos are more dangerous than most other common world creatures, but little danger to an experienced player. Mostly only newbies fall prey to them these days far as I can tell.
1All references to carp in the previous paragraphs apply to all large fish, not just their designated public relations fish
P.S. why did the chicken cross the road? To get to the other side! HAHAHAHAHAHA! (well you didn't say anything about cliche non-DF jokes)