Their zakat is necessary when the male and female horses are fed together for breeding in the fields. Zakât is not necessary if they are intended for transportation or for carrying things. Zakât is not fard for a person who has only male horses [stallions]. For, he cannot breed them. If they are kept for commercial purposes, one pays their zakât as commercial property. Zakât is not paid for mules and asses not intended for trade, even if there are a legion of them. There is not a nisâb for horses. One mithqâl of gold is given for each horse. One may as well calculate their value and give onefortieth of their value in gold if their value equals the amount of nisâb in gold. Also for camels, cattle and sheep that are given as zakât, their equivalent in gold can be given.
Source : Endless Bliss