Granted. Intense selective breeding creates a dwarf cow to sell as designer pets. The process is excessively cruel and incestuous, and the dwarf cows are in constant pain, as well as stupid and easily angered. Animal rights groups see the creation of dwarf cows as more inhumane than beef production.
Granted. They are immediately more endearing than any pet anyone has ever had... But by 3 years of age they are more delicious than any food anyone has ever eaten.
Mini-Cows will never become house trained so you become accostomed to a warm-squishy cow patty gushing between your toes when you walk to the bathroom late at night.
Then buy some cows, breed them, and selectively breed them until they're really small. But don't forget about secondary characteristics like aggressiveness, etc. Don't breed them to be a smash-faced pug with breathing problems.
Better hurry, though, since culture might change and suddenly domesticated animals like cows are recognized as people with rights and 20th century burger-eating culture becomes taboo.