Would AI assume horns are a sort of decorative headpiece, like a hair ribbon or bow?
Depends on the set you trained with. This specific set? Yes.
F'rex suppose you train your AI on the aforementioned Boris Vallejo. If you draw a guy, he's probably going to be huge and muscled by default. If you give him horns, the AI will probably put a horned helmet on him rather than giving him horns on his skull because that's where most of the horns in his art come from. Even if you don't ask for a helmet, in the AI's mind horns and helmets go together on men.
Train on World of Warcraft art instead, though, and it will have a solid understanding of guys with horns growing from their skulls, and will probably avoid putting helmets on them unless specifically told to because relatively few WoW men wear them and even fewer with horns. It will conclude that horns and helmets are essentially contradictory and it's one or the other.
That's why this specific image was a touch tricky and needed twelve renditions, since it was trained on real people it had very few models with horns in the first place and they were all wearing horned headbands so it kept wanting to add a headband. I had to essentially brute force it by putting a very high priority on horns in the positive prompt, and also putting a very high priority to avoid headbands in the negative prompt to get it to understand what was needed. I also had to put a high priority on avoiding cat ears because cat-eared headbands look a lot like horned headbands at a glance, and, I suspect, are far more common than horns, so a solid half of my first rendition actually had the horns as cat ears.
This was a washout from the first set, as an example. You can see the AI was thinking "A headband goes there" but also had instructions not to put in a headband, so it recolored a strip of her hair black where a headband would go.
This incidentally was my second choice. I actually like the face better but didn't want to do a third set to fix the hair overlapping one of the horns so it just barely didn't make the cut. It also has the AI dithering on the subject of headbands and coloring a strip of hair where a headband would go.