Tauros breed with them, most likely. Or if these field-guide pokemon follow the breeding rules from the games then any male pokemon of the correct egg group (field/ground) can breed with them and produce miltank.
These are absolutely gorgeous and fun to read, though I do have to add that i /think/ milk is not stored, but is "produced on demand". However, i may be wrong, and with the internet the way it is, I don't know if I want to go check.
A pink cow! Yay Say, do you have Milka where you live or is that a local thing? It's a chocolate producer and its trademark is a purple cow. Would there be a pink cow (Miltank) on the covers of the Moomoo chocolate (Please say yes)?