Some things are the same in plural as singular. "Deer" is one of them. Fish (one fish, five fish), sheep (one sheep, one hundred sheep) are other common ones.

Dante: In addition to Amanda's fine answer to your question, you should keep in mind that English is a "flexible" language. There are rules, and there are exceptions to the rules. Beyond even that, there is a common habit amongst Americans to break all rules. One example is to talk "Baby Talk". A small Child, not knowing proper grammar, might use the word "Deers" to talk about the Deer they see. In a similar fashion, Adults sometimes adopt "Baby Talk" as a Slang usage. For example, the plural of Goose is not Gooses, but GEESE. In spite of that, an American might decide to appear "cute" and begin talking about the Deers----Gooses----Goosies-----Sheeps----Sheepses----Horsies----Cowsies---Fishies....and so forth. Just remember that in writing English, follow the Grammatical Rules. In casual conversation, there are occasions when a very "familiar" or "intimate" attitude is adopted. Perhaps the reason for this is that many people adopt and express a tender, and loving attitude about animals. Sometimes this can become quite comical. For example, a baby dog is a "Puppy". To be "cute" however, it is not uncommon for an American to say something like: "Oh! Look at the little "Poopy!" Poopy is a reference, by spelling with Double "O's" to "Poop" which comes from the dog's bottom or behind and must be cleaned up. Just remember that not all words in English are made plural, by adding "s".

"deer" and "sheep" don't add -s