Mbh 1.172.6b

They may be lean-fleshed but they tire not, nor doth their speed suffer on that account. In days of yore the thunderbolt was created for the chief of the celestials in order that he might slay (the Asura) Vritra with it. But hurled at Vritra’s head it broke in a thousand pieces. The celestials worship with reverence those fragments of the thunderbolt. That which is known in the three worlds as glory is but a portion of the thunderbolt. The hand of the Brahmana with which he poureth libations on the sacrificial fire, the chariot upon which the Kshatriya fighteth, the charity of the Vaisya, and the service of the Sudra rendered unto the three other classes, are all fragments of the thunderbolt. It hath been said that horses, forming as they do a portion of the Kshatriya’s chariot, are, on that account, unslayable. Again horses which form a portion of the Kshatriya’s chariot, are the offspring of Vadava. Those amongst them that are born in the region of the Gandharvas can go everywhere and assume any hue and speed at the will of their owners. These horses of mine that I give thee will always gratify thy wishes.”