We can assume that regular animals can and have fallen into the underground. This is technically proven in ut yellow (yes ik it's not canon but still) where a monster has pet snakes. By this logic, we can assume cows are in the underground in an area closed off from the regular game.
True, but barely anything comes down from the mountain, can be confirmed that it took Asgore centuries to get the 6 human SOULs, and the mountain is uninhabited. Also, no animal can survive the 1000 foot fall.
Well, then the mountain is still uninhabited, and like all human wildlife, they eventually die. So what else is there to substitute milk? We know that Toriel is a goat-
