Valāhaka is not explained in any dictionary I have seen, and I have not been able to find it in pre-Buddhist texts. Trace to the root val (to “turn” or to “arc”). Appended with āha (“sky”) we have valāhaka, “one who arcs over the sky” like a constellation (cf. Sanskrit valana). This is the name of the horse of the universal monarch (MN 129:37.2), and metaphorically the “thunderclouds” that rumble like horse’s hooves. The link between horses and clouds recalls the Vedic Maruts, boisterous gods of the thunderclouds, who roamed about roaring in their chariot drawn by horses (Rig Veda 1.39.6, 1.87.4, 1.88.1, 5.42.15, etc.) or on horseback (RV 5.61.2), which are compared to the rainy clouds (RV 5.53.7, 8.7.28). Other deities also have horses associated with the thundercloud. The twin Asv́ins (“horsemen”) have horses that “thunder” (RV 5.73.7). The horses of Parjanya (Pali pajjuna) are “rain-bearing messengers” (RV 5.83.3). Bṛhaspati has horses “like a ruddy cloud” (RV 7.97.6). Pegasus, who was said by Hesiod to be the bringer of thunderbolts to Zeus, harks back to the same mytheme.

The “host” (kāya) is the four types.

Abbha (“stormy”) by itself just means “cloud”. In the Rig Veda they’re associated with wind (10.68.5), rain (10.77.1), and lighting (9.76.3), and in the Suttas they are described as “blue” (Thag 1.13:1.1).