This bit of trivia brought to you by the ongoing horrorshow that is my buffer.
“Bug” derives from a couple of old British languages (Middle English ‘bugge,’ Old Scottish ‘bogill,’ some incomprehensible Welsh thing) which basically just means “creepy thing.” The modern word “bug” is derived from this, and refers to any sort of creepy crawly thing, including spiders, centipedes, and other non-insects. Some variations on “bug” were archaically used to refer to goblins (i.e. fairies that like to fuck with people), so a “bugbear” is a scary bear, or even specifically a goblin-bear, in late medieval English.