The Vastaya are a race in League of Legends. They are chimeric creatures of Runeterra whose lineage contains both human blood and the spiritual magic of a pre-human race. From the ferocious, bestial power of Rengar to the foxlike allure of Ahri, vastaya can look considerably different from one another, but they all share animal and human characteristics.
It's a meaningless race tag for the purpose of danbooru, because it has no clear definition and basically means "animalistic traits, but for league of legends". While Yordles have a specific appearance and ear-shape, vastaya are way to broad. Nobody would search for it, when he wants ahri just to (sometimes) find images of rengar or others.