A strict, bastard-DMy reading would say that a minotaur monk can't use the monk's Martial Arts ability. Martial Arts says "When unarmed or wielding only monk weapons [...]". Minotaur says "You are never unarmed", and horns are not a monk weapon.
That would be a very dumb reading, but completely valid under the rules.
One word could fix it for RAW - add the word "simple" in the horns ability. Then, horns would read...
"...You are proficient with your horns, where are a simple melee weapon..."
and Martial Arts reads
"...and monk weapons, which are shortswords and any simple melee weapons that don't have the two-handed or heavy property..."
This is likely the Intent, and it'd be pretty easy to make that the RAW, so this would be one of those things the playtest-status is meant to suss out.
Actually, I might take that back...if you do that, you have a 1d10 finesse melee weapon and any monk who isn't a minotaur will feel pretty boned by that.
Maybe a better way to do that is to re-word horns to be "You are never unarmed, unless you choose to be."
That way, a monkotaur could choose to be unarmed to have access to martial arts, or choose to be armed to hit things with their head, and could basically swap that out whenever they wanted.
Which feels better to me than a 1d10 finesse melee weapon for monkotaurs that makes every dwarf and elf monk cry with their useless weapon proficiencies.
