Cows

From Animal Crossing - Wiki on Neoseeker

The Cow is a villager species that has appeared in every version of the game. Although they share a similar appearance to Bulls, they are different. The main attribute is that Cows are all female and have shorter horns on their head.

Trivia

There have been at least 2 Cows in every game of the series.
Patty is the only Cow to appear in all games of the series.

Villager Species

Name	Games	Birthday	Picture Inscription
Belle	Animal Crossing	N/A	N/A
Bessie	Animal Crossing	N/A	N/A
Patty	Animal Crossing Animal Crossing: Wild World Animal Crossing: City Folk	May 10th	"This month's Ms. Nintendique cover girl!"
Petunia	Animal Crossing	N/A	N/A
Tipper	Animal Crossing: Wild World Animal Crossing: City Folk	August 25th	"I know you'll treasure this forever."