Utbah heard the Messenger of Allah ﷺ say: Do not cut the forelocks, manes, or tails of horse, for their tails are their means of driving flies, their manes provide them with warmth, and blessing is tide to their forelocks. (Using translation from Abū Dāʾūd 2542)   

أَنَّهُ سَمِعَ رَسُولَ اللهِ ﷺ يَقُولُ لَا تَقُصُّوا نَوَاصِيَ الْخَيْلِ وَلَا مَعَارِفَهَا وَلَا أَذْنَابَهَا؛ فَإِنَّ أَذْنَابَهَا مَذَابُّهَا وَمَعَارِفَهَا دِفَاؤُهَا وَنَوَاصِيَهَا مَعْقُودٌ فِيهَا الْخَيْرُ