Etymology
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walrus (n.)

"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.

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sea-horse (n.)

late 15c., "walrus" (apparently), from sea + horse (n.); compare walrus. Also in heraldry as a fabulous animal with the foreparts of a horse and the tail of a fish. Main modern sense in zoology is attested from 1580s.

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sea-dog (n.)

1590s, "harbor seal," from sea + dog (n.). Also "pirate" (1650s). Meaning "old seaman, sailor who has been long afloat" is attested by 1823. In Middle English sea-hound was used of the walrus and the beaver.

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sward (n.)
"grass-covered ground," c. 1300, from Old English sweard "skin, hide, rind" (of bacon, etc.), from Proto-Germanic *swarthu- (source also of Old Frisian swarde "skin of the head," Middle Dutch swarde "rind of bacon," Dutch zwoord "rind of bacon," German Schwarte "thick, hard skin, rind," Old Norse svörðr "walrus hide"). Meaning "sod, turf" developed from the notion of the "skin" of the earth (compare Old Norse grassvörðr, Danish grønsvær "greensward").
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whale (n.)

"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.

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seal (n.2)

"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."

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