Acid Armor has a pretty specific flavor to it, leading to a relatively narrow distribution with predictable reasons for why a given Pokemon learns it. You got blobs like Grimer and Litwik, you got gastropods like Goomy and Shelmet, you got jellyfish like Tentacool and Frillish, and you got things that melt like Vanillite and Vaporeon. There are only two exceptions to this. First there's Skrelp, who kinda sorta fits the flavor with its Poison/Water typing, and it's an egg move so the flavor doesn't need to fit
that much. Second,

there's Glimmet, who learns the move through level-up.