Gibbon's footnotes are justly famous. Here's a footnote to a passage describing how Commodus wastefully slaughtered rare animals in the arena.
1. Commodus killed a camelopardalis or giraffe, the tallest, the most gentle, and the most useless of the large quadrupeds. This singular animal, a native only of the interior parts of Africa, has not been seen in Europe since the revival of letters, and though M. de Buffon has endeavoured to describe, he has not ventured to delineate, the giraffe. (v. 1 p. 107)