Yes, RHEAS is a Scrabble word!

Letter values in Scrabble US: R 1, H 4, E 1, A 1, S 1.

Is it a valid word?

RHEAS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

fertility goddess in ancient Greek mythology; wife of Cronus and mother of Zeus; identified with Roman Ops and Cybele of ancient Asia Minor
smaller of two tall fast-running flightless birds similar to ostriches but three-toed; found from Peru to Strait of Magellan
larger of two tall fast-running flightless birds similar to ostriches but three-toed; found from Brazil to Patagonia

Also: pterocnemia pennata, rhea americana.

Not playable in Scrabble US: NANDU.