The pronoun is "it" and the antecedent is "a miniature glass hippopotamus".
Extra information:
The word antecedent is composed out of two Latin words: ante and cedere, and it literally means: the "preceding", meaning it refers to the word that precedes the pronoun, that the pronoun refers to. This is not important for the question, but it is important to remember that the antecedent is always in front of the pronoun!
