User blog comment:BannedLagiacrus/Monster Appreciation Week: Hellblade Glavenus/@comment-33581177-20170221170226/@comment-26034899-20170221172331

You unlock Deviants by clearing certain Hunter Hub quests that require you to hunt the normal versions. For instance, you can unlock Snowbaron by doing the "Snow with Occasional Lagombi" quest. They all require you to hunt just one monster to succeed, so they shouldn't be too hard to find.