What is the plural possessive form of "deer"?

deer's

Since the plural of "deer" is "deer," the possessive plural is "deer's".

In the case of a plural possessive where the plural doesn't already end in -s, the plural possessive is formed by adding the -s. Another example would be, the plural possessive form of "man" is "men's".

Example using "deer's":

Since their owner was an avid fan of the movie, all the deer's names were Bambi.