As nouns the difference between rhea and rea

is that rhea is a large flightless bird of the genus Rhea, native to South America while rea is alternative form of lang=en.

As a proper noun Rhea

is a Titan, the daughter of Uranus and Gaia, the mother of Demeter, Hades, Hera, Hestia, Poseidon, and Zeus.

As an initialism REA is

request for an equitable adjustment, a term used in contracting with government entities in the United States to indicate that compensation is sought for unexpected costs arising in the fulfillment of a contract.