"large pinniped carnivorous mammal" (the males are noted for their enormous tusk-like canine teeth), 1650s, from Dutch walrus, which was probably a folk-etymology alteration (by influence of Dutch walvis "whale" and ros "horse") of a Scandinavian word, such as Old Norse rosmhvalr "walrus," hrosshvalr "a kind of whale," or rostungr "walrus." Old English had horschwæl, and later morse, from Lapp morsa or Finnish mursu, which ultimately might be the source, much garbled, of the first element in Old Norse rosmhvalr.
"animals of the mammalian order Cetacea," Old English hwæl "whale," also "walrus," from Proto-Germanic *hwalaz (source also of Old Saxon hwal, Old Norse hvalr, hvalfiskr, Swedish val, Middle Dutch wal, walvisc, Dutch walvis, Old High German wal, German Wal), from PIE *(s)kwal-o- (source also of Latin squalus "a kind of large sea fish"). In popular use it was applied to any large sea animal. Phrase whale of a "excellent or large example" is c. 1900, student slang. Whale-oil attested from mid-15c.
"fish-eating marine mammal with flippers; any pinniped not a walrus," Middle English sele, from Old English seolh "seal," from Proto-Germanic *selkhaz (compare Old Norse selr, Swedish sjöl, Danish sæl, Middle Low German sel, Middle Dutch seel, Old High German selah), a word of unknown origin, perhaps a borrowing from Finnic.
Seal point "dark brown marking on a Siamese cat" is recorded from 1934, from the resemblance to the color of seal fur; compare seal brown "rich, dark brown color," which is attested by 1875. Old English seolhbæð, literally "seal's bath," was an Anglo-Saxon kenning for "the sea."