1520s, cubbe "young fox," of unknown origin, not recorded in Middle English; perhaps from Old Irish cuib "whelp," or from Old Norse kobbi "seal." Extended to the young of bears, lions, etc., after 1590s. The native word was whelp. Cub Scout is from 1922.
"small, enclosed space," 1825, the first element possibly from a diminutive of cub "stall, pen, cattle shed, coop, hutch" (1540s), a dialect word with apparent cognates in Low German (such as East Frisian kubbing, Dutch kub). Or perhaps it is related to cuddy "small room, cupboard" (1793), originally "small cabin in a boat" (1650s), from Dutch kajuit, from French cahute. OED calls it "a nursery or children's name."
breed of dog, a kind of sheep-dog much esteemed in Scotland, 1650s, of uncertain origin. Possibly from dialectal coaly "coal-black," the color of some breeds (compare colley, "sheep with black face and legs," attested from 1793; Middle English colfox, "coal-fox," a variety of fox with tail and both ears tipped with black; and colley, Somerset dialectal name for "blackbird"). Or from Scandinavian proper name Colle, which is known to have been applied to dogs in Middle English ("Ran Colle our dogge, and Talbot, and Gerlond" [Chaucer, "Nun's Priest's Tale"]). Century Dictionary cites Gaelic cuilean, cuilein "a whelp, puppy, cub." Or perhaps it is a convergence of them. Border-collie (by 1894) was so called from being bred in the border region between Scotland and England.