One who undertakes the charge of a young man of rank on his travels. It was once customary to lead muzzled bears about the streets, and to make them show off in order to attract notice and gain money.
“Bear! [said Dr. Pangloss to his pupil]. Under favour, young gentleman, I am the bear-leader, being appointed your tutor.” —G. Colman: Heirat-Law.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894