The Re’em is an extremely rare golden-furred giant ox.
- Re'em blood gives the drinker super strength. However, since demand far exceeds supply, it is rarely found on the open market (FB).
Commentary
Etymology
According to the NSOED's historical information about the word 'unicorn', "re'em" was Hebrew for 'wild ox'.
Notes
The Re'em is not featured in any of the books other than Fantastic Beasts and Where to Find Them.
JKR borrowed the idea of the Re'em from ancient writings, although she created her own description of this mysterious beast.
It is quite interesting that the Re'em is golden-furred, reminiscent of the "Golden Fleece" sought by the hero Jason in Greek mythology. However, that story is usually concerned with a golden ram (sheep), not an ox.
From 
The Truth About Unicorns by James Cross Giblin:
"In the original version, the Hebrew writers referred seven times to a powerful animal called the re'em. One of these references appeared in the description of Joseph.... Not knowing what animal the Hebrews had in mind, the authors of the Septuagint translated re'em as monoceros, the Greek word for unicorn. "There's no reason to believe, however, that the Hebrews thought of the re'em as one-horned. Some later scholars argued that it was probably the African antelop, the oryx. They pointed out that the Arabic word for oryx was the similar (word) 'rim'" (page 41).
The reference to Joseph is from the Biblical book of Deuteronomy: "His Glory is like the firstling of his bullock, and his horns are like the horns of unicorns...." (Deut. 33:17). (Thanks to Pigwideon on the Lexicon Forum for this information)
From the Web
"Re'em" from Harry Potter Wiki