"We have a cat at home."
Should also accept 有只猫， 只猫 is like a cat, and 一只猫 would rather be like one cat
In this case the character 只 is the classifier for animals. In traditional Chinese it is a different character: 一隻貓
For Mandarin(and apparently Cantonese隻) 只(zhī) is a classifier for: one of a pair (hands 手, legs), birds 鸟, and some animals (cats 猫, etc.)