curb (n.)
controlling chain or strap passed under a horse's jaw; check, restraint
AYL III.iii.73 [Touchstone to Jaques] the horse [hath] his curb
Cor I.i.68 [Menenius to Citizens, of Rome's power] cracking ten thousand curbs / Of more strong link asunder than can ever / Appear in your impediment
TNK V.iv.74 [Pirithous to Palamon, of Arcite's horse] When neither curb would crack