They are a specific thing in the fiction, more than a pony with a horn and wings.
In short, alicorns only come in one of the three colors. Each color has a different forte. It's because of what made them / how they're made.
Fallout: Equestria Wiki (Spoilers, if you're… like, at least before ch. 20 or 30? I don't remember exactly. Why would I. Anyway, there's how alicorns work and what the being leading them is.)
The tags seem acceptable IMO.