The regular way to form plurals in English is to add an ‘s’. However, there are some exceptions and deer is one of them. The plural of deer is deer: I saw several deer in the park. Other animal names that don’t take an ‘s’ in the plural form are fish, bison, and sheep.
The whole subject of plurals in English is so littered with inconsistencies that there is even a little poem about it:
We’ll begin with a box, and the plural is boxes;
But the plural of ox should be oxen, not oxes.
Then one fowl is goose, but two are geese;
Yet the plural of moose should never be meece.
You may find a lone mouse or a whole lot of mice,
But the plural of house is houses, not hice.
Vivatraining 