This is a counter for "one small animal" or "one insect." It is often used for pets like cats, dogs, or fish, and for smaller creatures. Note the pronunciation change from "ひき" to "ぴき" for one.

小動物や昆虫を「一匹」数える助数詞。猫、犬、魚などに使います。発音は「いっぴき」です。