The front horse or leading horse in a team of draft animals pulling a wagon, plow, or carriage.
From 'fore-' meaning front, combined with 'horse' from Proto-Indo-European roots. In medieval and agricultural contexts, the forehorse was the most visible and important animal in a team.
The forehorse was the most skilled, intelligent horse in a team because it had to respond to the driver's smallest cues and guide the other animals—often they were specifically trained, preferred animals worth more money.
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