pin-horse
/ˈpɪnhɔːs/(also pin-hoss, pin-'oss) English regional. Now rare
noun
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.