The excess energy from Mega Evolution spilled over from its mouth, breaking its jaw. It spews endless blizzards.
– Pokémon Sun Pokédex
Mega Glalie is one of the ugliest mega Pokemon imo. I mean, Glalie is already pretty ugly, the dislocated jaw and the extra facial hair don’t help. Though reading its Pokedex entries, I feel bad for it. The Moon Pokedex says “It envelops prey in its mouth, freezing them instantly. But its jaw is dislocated, so it’s unable to eat them.”
Stat-wise, it gains Att, Sp Att, and some Speed.