Q. What is this mystery plant? Is it Amaranthus?
The plant is indeed an an amaranth; it is commonly known as Amaranthus 'Elephant Head.' There seems to be some tiny bit of confusion re the correct nomenclature. It was known as Amaranthus gangeticus "Elephant Head,' but apparently the correct name is Amaranthus tricolor (just like almost every other amaranth except for the 'Elephant Head' cultivar). If you want to get more of this (though from the pix I find, it looks like there could be too much of a good thing) Call it Elephant's Head, and probably buy seeds and grow it that way; it is, by the way, an annual.