匹

Outer wrapping 匚 — the open-box radical (Kangxi #22), a frame open to the right. Indexing radical of 匹. (Visually near-identical to 匸 #23 with the same shape; modern dictionaries assign 匹 to 匚.) The frame outlines a folded length of cloth or a paired roll, source of 匹's measure-word use for cloth bolts and matched horses.

Inner two strokes — graphically 儿 (a person's legs / paired uprights). In 匹 they represent the two layers of a folded bolt of cloth tucked inside the frame; the original picture is a length doubled back on itself, source of both 'a bolt of cloth' and 'a matched pair' (a horse paired with another, hence 一匹马).