According to this http://ffxiv.gamerescape.com/wiki/Yak_Milk, supposedly, it is... however, I asked a guy from my static and he claimed Yak Milk wasn't on his hunting list despite having his retainer at level 60 (not sure if he's serious or trolling me xD).
Could somebody clarify? I'm pretty interested in this ingredient because it's asked in so many culinarian recipes and killing wooly yaks is eternal.