A preliminary report suggests that, at least in Adventure Mode, these creatures may have taken the initiative to become the new King of the Beasts. While the mighty elephants appear to have had their aggressive tendencies curbed somewhat, the unicorn, once as equally peaceful when undisturbed, now appears to have become a killer, attacking peasants, children and elves without any warning and, apparently, without any provocation. Wildlife preservationist groups warn players to not be hasty and that not all elves are trustworthy sources of information, but this reporter isn't going to be visiting any more joyful forests anytime soon (at least, not without some good old fashioned metal around).
In Fortress mode, it appears that the unicorn does not actively seek to murder dwarves without provocation, but it is possible that something will cause them to go on a rampage. It's probably worth the risk if you like hunting though, because items from unicorns are ten times as valuable than items from more boring animals like deer.
Unicorns cannot be tamed unless you manually mod its game data files to include: [PET] or [PET_EXOTIC]. This is all the more tempting since their petvalue is enormous.
[CREATURE:UNICORN] [NAME:unicorn:unicorns:unicorn] [TILE:'U'][COLOR:7:0:1] [MODVALUE:10] [PETVALUE:1000] [PREFSTRING:horns] [LARGE_ROAMING] [GENPOWER:4] [BENIGN][MEANDERER] [BODY:QUADRUPED:TAIL:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN:HEAD_HORN:MOUTH] [BODYGLOSS:HOOF][MOUNT] [POPULATION_NUMBER:15:30] [CLUSTER_NUMBER:3:7] [SIZE:9] [ATTACK:MAIN:BYTOKEN:HEAD_HORN:stab:stabs:1:6:PIERCE][ATTACKFLAG_WITH] [ATTACK:SECOND:BYTYPE:STANCE:kick:kicks:1:2:BLUDGEON][ATTACKFLAG_WITH] [FAT:5] [DIURNAL][GOOD] [BIOME:FOREST_TAIGA] [BIOME:ANY_TEMPERATE_FOREST] [BIOME:ANY_TROPICAL_FOREST] [BIOME:SHRUBLAND_TEMPERATE] [BIOME:SHRUBLAND_TROPICAL] [STANDARD_FLESH] [HOMEOTHERM:10067] [LAYERING:100] [SWIMS_INNATE][SWIM_SPEED:2500]