How do you spell deer singular and plural?

If all of the animals are of the same species the plural is 'deer'. If you are emphasizing that the collection of animals is of different species you would use 'deers'.

Examples: Moose, elk, mule, whitetail, and key are deers. Santa has a team of reindeer.

Similarly, a mouse, some mice and several types of mouses are all correct.

Ain't English fun?