The Aberdeen Bestiary describes real and imaginary animals. It was written in England around the year 1200. This online version is presented by King's College, University of Aberdeen.
A collection of translations and retellings by folklorist and retired professor D.L. Ashliman, hosted by his former employer, the University of Pittsburgh.
Other sites
These sites are not affiliated with a university or government agency, but are nonetheless reliable.
Theoi is an encyclopedia of Greek gods, heroes, and other beings, accompanied by examples of how they have been depicted in art. Information about the site's author is not available, but articles frequently include citations.