head, counter for animals
疋 centers on counting animals, especially smaller ones, and can also refer to a head or bolt of cloth. The 'head' meaning is an extension of the counter use, treating each animal as a single unit.
疋 is a pictograph of a foot or leg, which later came to be used as a counter for animals (originally for cloth bolts, then extended to animals). The exact historical development is uncertain.
Picture a foot 疋 stepping to count animals one by one, like a farmer counting sheep.
For ヒキ, imagine a hickey on your foot from counting too many animals: hickey -> ヒキ.