Cow

Cows are a type of NPC animal found in The Sims 2 for Nintendo DS and PSP. Cows also appear as a part of the "Farm Fresh Folk Set" on The Sims 3 Store.

The Sims 2 (Console)
Cows are featured in The Sims 2 for Nintendo DS. They are found in the field outside of Strangetown, near Jebidiah S. Jerky's home. They can be interacted with in three ways; Players can either milk them, or tip them over. They can also give the cow a gift, but only chocolate seems to have an effect. After the Sim has tipped the cow over, it will simply come back up. In the field, there's a red coloured bull. No interactions are allowed with this animal. If a cow is milked after it has been tipped over it will produce a milkshake instead of a glass of milk. However, if the cow has been given chocolate it will produce chocolate milk, or chocolate milkshake (if it has been tipped over). This is only a feature though and is profitable nor more effective in filling the need bar.

Cows also appear in The Sims 2 for PlayStation Portable. They appear in the "Kine Dairy" building in Deadtree, but cannot be interacted with.

Cows and Bulls
Cows are the main animals on the farm but there are some Bulls. Bulls cannot be milked (obviously) and cannot be interacted with. To spot one you need to go to Jebidiah's Home. There you will see Red looking cows and they are the Bulls.

The Sims 3
Cows returned to The Sims 3 in the "Milkin' It Dairy Corral" premium content included in the "Farm Fresh Folk Set" from The Sims 3 Store. SimGuruTaterTot has shared some information about what this cow can do:

Abilities

 * Sims can milk cows to get milk.
 * Milk can fulfill Sim's hunger need.
 * Feeding the cow vegetables will make it produce soy milk, which will also revitalize the Sim's age (similar to Life fruit and ambrosia).
 * Feeding the cow chocolate will make it produce chocolate milk, which will also boost fun.
 * Sims can turn milk into cheese.
 * Sims can tip the cow and make it produce a milkshake.
 * Sims can play Tic Tac Hoove with a cow to increase the Logic skill.