Hippos seem to be peaceful, friendly and not very intelligent river creatures with a strong tendency to make loud noises while ... reproducing. But this is not quite correct. Every dwarf who was courageous enough to settle near the ever growing colonies of the hippos will soon describe them as the monstrous beasties they are: stealthy, bloodthirsty killers who will assassinate every innocent dwarf on sight!
 Bloody Rites of the Hippos
Hippos travel through their territory as a swarm, harmless to most other animals. When they meet similar semi-intelligent groups like immigrants or invaders, suddenly an individual Hippo's thirst for blood awakens. After the herd massacres whoever they stumble across, this warlord hippo (who is given a cool name) will fight any humanoid lifeforms to death, actively supported by the brave rest of the herd.
 How to fight Hippos
In one word: Don't. Unless you have a strong army of Spear- and Marksdwarves, hippos will pulverize your dwarves. Just set up long lines of spiked hippo traps or dig channels around your entrance. The biggest problem is water. Dwarves will drink the pesky stuff if there is no alcohol, making themselves living targets with a "drown me" sign on them.
 How to fight Hippos in Adventure Mode
So you've discovered a river! Good for you! Hippos are one of the good reasons to not throw off those clothes and go skinny dipping just like that. They may, at first, appear like they are horses drowning. Don't be a hero and dive in after them, you want to make sure first. Then, if you aren't accompanied by numerous bowmen or crossbowmen, and they are indeed hippos, it would be a good time to leave. Otherwise you can attack them, but be prepared for fun.
[CREATURE:HIPPO] [NAME:hippo:hippos:hippo] [TILE:'H'][COLOR:7:0:0] [LARGE_ROAMING] [AMPHIBIOUS] [MODVALUE:3] [GENPOWER:3] [PREFSTRING:strength] [IVORY] [DIURNAL] [POPULATION_NUMBER:15:30] [CLUSTER_NUMBER:3:7] [BENIGN][MEANDERER][NATURAL][PET] [PETVALUE:400] [BODY:QUADRUPED:TAIL:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN:MOUTH] [SIZE:12] [MAXAGE:40:50] [ATTACK:MAIN:BYTYPE:MOUTH:bite:bites:1:6:GORE][ATTACKFLAG_CANLATCH] [CHILD:5][CHILDNAME:hippo calf:hippo calves] [FAT:10] [BIOME:RIVER_TROPICAL_SALTWATER] [BIOME:RIVER_TROPICAL_BRACKISHWATER] [BIOME:RIVER_TROPICAL_FRESHWATER] [BIOME:LAKE_TROPICAL_SALTWATER] [BIOME:LAKE_TROPICAL_BRACKISHWATER] [BIOME:LAKE_TROPICAL_FRESHWATER] [STANDARD_FLESH] [HOMEOTHERM:10067] [SWIMS_INNATE][SWIM_SPEED:2500] [MUNDANE]