As someone who can milk livestock, I'd say that it is probably drinking milk from any other species apart from ourselves. I imagine some Biblical figure sitting on a rock, thinking, "Hmm, that calf/lamb/kid seems to enjoy that milk, so I'll try some, too!"
Despite this, we wouldn't think to drink human milk, and find it strange to think that, in other countries, they drink more goat/sheep milk than cows'.