As a modifier. Designating a packhorse or (occasionally) other working animal used to carry loads; (also sometimes) designating a vehicle used to…

A pack-saddle. Only in combinations, as bat-needle n. a packing-needle (obsolete). bât-horse n. (French cheval de bât) a sumpter-beast, a horse…

(also figurative)

a mule harnessed to the pole of a wagon.

(cf. lead-horse n.).