A Cow is an animal that can be purchased at Marnie's Ranch for data-sort-value="1500">1,500g, after owning a Barn. Cows come in two varieties: White and Brown. When purchasing a cow, the game decides randomly whether you will get a white or brown cow (50/50 chance).
Cows mature after 5 days, at which point they will produce Milk every morning, unless they are grumpy, in which case milking them will result in the dialogue "[Cow Name] has no milk right now". Once sufficient friendship and happiness is reached, they will produce Large Milk. Milk can be collected using a Milk Pail. Milking a cow increases friendship by 5 points.
Cows can be sold. The selling price is dependent on how many hearts the animal has (data-sort-value="1950">1,950g max).
To sell a coop or barn animal, right-click the animal after petting. The friendship/mood interface appears. Move the cursor over the gold coin button at the right to see how much you can sell the animal for. Click that button and the confirmation that follows to complete the sale.