A ghost, a bugbear. A caddis is a grub, a bait for anglers.
Poor Mister Leviathan Addy! Lo! his grandeur so lately a sun, Is sinking (sad fall!) to a caddy.
Peter Pindar: Great Cry and Little Wool, epistle 1.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894