The genus Amaranthus is in the plant family Amaranthaceae. Plants of the World Online lists 91 species in the genus Amaranthus as of 3-1-21 when I last updated this page.
Many Amaranthus species share characteristics and uses with the Celosia genera, a member of the same family.
Various species in this family are grown for their ornamental flowers, some hanging downward and some resemble large “Celosia-like” flowers. The leaves and seeds are also used as a vegetable in some species. The grain is very nutritious and does not contain gluten. The leaves are an excellent source of vitamin A, C, calcium, manganese, and folate.
I am not going into all the history, at least not unless I get bored later. You can easily read more about the Amaranthus by typing it into your browser. Wikipedia has lots of information.