The plural of deer is deer.
Example 1: Deer are found in all continents except Antarctica and Australia.
Example 2: Deer are excellent jumpers and swimmers.
The word “deer” can be either singular or plural depending on the context, and thus it is called an irregular plural.
You would say “one deer” and “100 deer,” but you would never say “100 deers.”
It does appear that many of the words that are identical in both singular and plural refer to animals, and specifically those that are hunted and/or herded. As to why this particular irregular plural exists may be subject to conjecture.
In rare, nonstandard, contexts, the plural form can also be deers (dated or nonstandard).
Example 3: The hill riding was of the roughest, and the cattle were wild as deers and as agile.
Example 4: Uncertainly, her head drawn back as a deers is when surprised, she glanced about her.