What about a "Fluttershy" Alicorn, with a non-lethal full-body paralysis breath attack of some sort (making outright deadly to fight against 1 on 1), that is also non-immune to some of it's own non-lethal syndromes, like getting mild paralysis/unconciousness from it's own blood (meaning if somepony was lucky enough not to get paralyzed, or you gang up on them, they go down easily).
Or for laughs a "Rarity" Alicorn with a lot of graspers, weapons and dangerous breath, that gets unconciousness from it's own blood.
Also "Pinkie Pie" Alicorn: Pony with pathetically weak melee attacks and body, but with 60 different variants of breath attacks to use, with effects varying from "mild numbness in left pinky finger" to "unstoppable contagion of necrosis, vomiting, bleeding, blisters and delayed lethal paralysis through-out entire fort"
(Arguably this would make them the most fun as well)