It's a structure that blocks the flow of water, often used for irrigation, flood control, or generating hydroelectric power.
The word you might be thinking of is "dam" which is a female animal, specifically a female mammal, that gives birth to live young and nurses them. This term can be used for various animals, including cows, goats, sheep, horses, pigs, and more.
So, depending on what you were looking for, the answer could be:
* None if you were asking about a farm animal.
* Many if you were asking about a female animal that gives birth to live young.