noun
gelding; castrated horse
Refers specifically to a castrated male horse. A technical term used in equestrian and veterinary contexts.
あのせん馬はとても穏やかだ。
That gelding is very gentle.
競走馬としてせん馬が使われることもある。
Geldings are sometimes used as racehorses.
牡馬 is a stallion (uncastrated male horse), while 騸馬 is a gelding.
牝馬 is a mare (female horse), while 騸馬 is a castrated male horse.
From Chinese 騸馬／骟马 (shànmǎ), meaning 'gelding'. The character 騸 means 'to geld'.