A counter for small animals (e.g., dogs, cats, fish); read as 'hiki'.
猫が二匹窓辺で寝ている。
Two cats are sleeping by the window.
子供が金魚を三匹買った。
The child bought three goldfish.
この庭には野鳥が何匹いますか？
How many wild birds are there in this garden?
犬が一匹、リードをつけて散歩していた。
One dog was being walked on a leash.
A counter for rolls of cloth (tan/bolt of cloth). Historically read as 'hiki' or 'ki' in some usages.
この着物の反物は一匹（いっぴき）で仕立てられている。
This kimono was tailored from one bolt of cloth.
古い商取引の記録に反物三匹と書かれていた。
Old business records listed three bolts of cloth.
An archaic counter formerly used for horses; rarely used in modern Japanese.
古文には馬二匹と記されていることがある（古語）。
In classical texts you sometimes find 'two horses' written using this counter (archaic).
A bolt or roll of cloth (the cloth roll itself).
その匹は上質な絹でできている。
That bolt of cloth is made of high-quality silk.