It is narrated from Sayyiduna Utbah bin Abd (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) forbade cutting the tails, forelocks and manes (neck hair) of horses, and said: "As for their tails, they are their means of driving away flies; as for their manes, they are their means of warmth; and as for their forelocks, goodness is tied to them."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14391