It vanished as quickly as it appeared. I don't have any item like that in my wardrobe, and I'm pretty sure it's not a stage in the zombie infection I signed up for (pretty sure, but not positive). It was cool, but I have no idea how it happened. Is there a random event or type of kiss or something that can make that happen? I would love to know.

It was one of the snowballs that can be thrown during winter. They make an appearance on your HA temporarily if somebody hits you with one.

You can see a list of what they look like here: link

Were you hit with a snowball? Snowballs are still active and can add overlays to avatars, and there is one that looks like a unicorn horn.

edit: whoops, was beaten to it :D

I completely forgot about snowballs. Don't remember seeing an alert about getting hit with one, but that's definitely what happened. Totally got hit by a "Magical Horned Snowball". Thanks you two!