This is the 482nd most frequent Japanese word.
匹
A counter for small animals.
In this sentence, "匹" is used as a counter for small animals such as cats.
この部屋には猫が三匹います。
There are three cats in this room.
Here, "匹" is applied as a counter for a dog, another small animal.
私の友達は犬を一匹飼っています。
My friend owns one dog.
In this context, "匹" counts the number of fish, following the general usage for small creatures.