In Greek, all vowels as well as the consonant r had punctuation marks called "breathings" in front of them: a "rough breathing" known as *daseia* "(" , or a "smooth breathing" known as *psili* ")".
These were dome away with by the late and unlamented
Andreas Papandreou in the name of "simplifying" spelling. IMHO, greek spelling is now much poorer. But I digress.
All Greek words starting with a letter that had a "rough breathing" got an "h", "s" or "v" when transliterated into Latin and eventually English. For example, the Greek (YPER became hyper or super, (OLOGRAMMA becomes hologram and so forth.
In a similar vein, (REA becomes
Rhea, just as (Rinokeros became Rhinoceros when transliterated :)