Sundancer is a rare that can be summoned in Bastion, and has a chance of dropping a mount. This NPC doesn't actually die, but forfeits on around 20%, so there isn't any lootable corpse. Will have to find a clever way of detecting these attempts

This is ~close to guaranteed for the person who spawns it, but other people can tag and get a chance on it to drop as well. I've personally seen 2-3 drops in a raid of 25+ people

Isn't this a guaranteed drop? wowhead comments suggest it has some peculiar conditions, but otherwise it shouldn't be a regular rare drop.

I'm not sure I understand how the drop works, but I suppose a possible way would be to detect combat and/or hostility change events (https://wow.gamepedia.com/Events#Unit), if those are triggered in the encounter. It's going to come with similar caveats as the Darkmoon Rabbit (#248), which isn't currently supported either.

Perhaps there's something else, too, but the only way to find out is to actually complete the encounter with /etrace enabled and filter out all the garbage events beforehand. It sounds like quite the headache, to be honest. It also doesn't look like wowhead would track the drop rate with how the mount is given out, so some estimates based on average drops/raid group might be needed to come up with an educated guess.