A milkmaid is a female who milks cows. She is not the one being milked. "Hucow" isn't a term I'd ever even heard before tonight. If you're interested in women who are treated like cows or are even part-cow, you might want to try using more than one word in your searches. "Cow" and "Girl" (or "Woman") is a good place to start, but you'll want to add more or else you're going to wind up with a bunch of cowboy (i.e. Wild West) hits.
I'm guessing you want more like this.
Tip: try mesu ushi (牝牛 or 雌牛 or メス牛), which means "female cow" in Japanese. You'll get a lot of positive hits for big-breasted women, but you'll get more lactating women than you were previously.