At the Dai Nagoya Building, there is a beautiful winter wonderland scene at the moment. It includes figures of deer (鹿). Most animal names are usual countable nouns (可算名詞). For example:
A cat.
Two cats.
Some cats.
An elephant.
Three elephants.
A lot of elephants.
Deer is also a countable noun. However, it doesn't use "s" for more than one deer.
A deer.
Five deer.
Many deer.
Other examples are:
A sheep.
One hundred sheep.
A few sheep.
A fish.
Twenty fish.
So many fish.