This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Re’em (Jewish)
Re'em is the biblical unicorn. The Bible mentions it seven times, always in the context of immense strength, indomitability and a long horn or horns rising in the direction of the sky. It is spoken of without explanation, the readers being expected to know the animal perfectly well.
The sources describe him as fierce, fast, and indomitable. Scholars argue about the number of his horns, some say he has one, like a unicorn or a rhinoceros. Some say two, and he could be related to the giant aurochs, a species of a wild ox that became extinct during the sixteenth century.