No, the word "zoo" is a noun. The form "zoo's" is the possessive form of the noun "zoo'. The apostrophe s ('s) at the end of the noun shows that a word in the sentence belongs to that noun (the zoo's gate or the zoo's keeper).

A pronoun is a word that takes the place of a noun in a sentence.

Example: The zoo's parking lot is very convenient. It is directly across from the main gate. (the pronoun "it" takes the place of the noun "parking lot" in the second sentence)