Definition of pin-horse in US English:

pin-horse

(also pin-hoss, pin-'oss)

noun

English Regional
rare
  • The middle horse in a team of three draught horses, or the third horse in a team of four, harnessed one behind the other.

Origin

Late 19th century; earliest use found in Edward Peacock (1831–1915). From either pin (although this is apparently first attested slightly later) or perhaps pin + horse.

Pronunciation

pin-horse

/ˈpɪnhɔːs/