Animal men who kill named creatures become "friendly". Maybe it's something to do with getting a name, I don't know, it doesn't matter. I've usually seen it happen when they manage to take down forgotten beasts, but today I discovered that it applies to other named creatures as well.
A giant toad, menaced by my hunters, had taken refuge in an olm man camp. It was basically the only wild animal on the map, so several hunters would approach it, take a few shots at it, forget what they were doing because OMG OLM MEN, shoot at the olm men for a bit, retreat and start over. The olm men put up a pathetic defense with their pitiable wooden spears and blowguns, but it was pretty clear they didn't stand a chance. Except of course that despite their unprovoked attack on a harmless indigenous tribe, these were technically still CIVILIAN dwarves, and tended to start running away every time they actually realized they were fighting something. The olm men were surviving just by taking arrows until the dwarves inevitably retreated.
Then, in a middle of an especially rowdy retreat, one of the hunters dropped a baby. Perhaps she'd laid the baby on the cave floor, to better aim her crossbow? Perhaps one of the olm men pushed her off-balance, causing her to lose her grip on the baby? We may never know. The baby was left in the middle of a pile of dead olm men, and it wasn't long before one of the wounded managed to limp over to the baby, clutching a bleeding wound in his side where a crossbow bolt still stuck out, and stab it dead with a spear. Around this time the toad waddled away from the olm men, causing the hunters to lose all interest in them.
So, obviously, in accordance with dwarven tradition, Frenzymires the olm man spearman is now listed as friendly in the units screen. In killing the infant, he also seized her citizenship. He's chilling out in the caverns with his two remaining olm man buddies, and the corpses of his non-remaining olm man buddies, with a bolt stuck in his liver. Living the dwarven life.