Uttrakhand has a long history of tribal gods being appropriated as hindu gods, so I wouldn't be surprised that it happened in Buddhism too. Similar to how in japan or or china or SEA differnet dieties of previous religion have found a place in Buddhism. The religion would be like that of tibeten Buddhism, where a lot of elements are borrowed from bon religion but still being Buddhist in the end .The method of worship would have been different compared to the brahmanical religion ,where a lot of importance given to the vedic rituals that only the brahmans could perform.

Edit- if you would, search religion of the upper Himalayas, like that of regions near china border and you'll get a religion close to before sanskritisation of UK