A goat. According to Vedic concepts, it is the steed of Pūṣā (Pusha) or Pūṣan (Pushan). Pūṣā, a form of the Sun-god, is addressed as ajāśva (ajashva), that is, he who has accepted aja or a goat as his steed much like a horse (aśva; ashva).
In Aśvamedha (Ashwamedha) yajña, mentioned in Vedas, a goat was carried ahead of the horses. It was thought that after the sacrifice of the horse of the yajña, the goat would show him the path to heaven.
In Atharvaveda, aja or the goat is described as playing a part in the final rites of the human-beings. During cremation, a goat is used to absorb the scorching heat of the funeral fire to ensure that the excess heat doesn’t scorch the body of the corpse immediately. That is why, in Atharvaveda, aja or goat has been mentioned as playing a part —
ajaṁ yantamanu taḥ samṛnvatām
athetarābhiḥ śivatamābhiḥ śṛtaṁ kṛdhi