Douglas Harper's Etymology Dictionary
c.1600, "kind of lobster," from sea + lion. Later the name of a fabulous animal (in heraldry, etc.), 1660s. Applied from 1690s to various species of large eared seals. As code name for the planned German invasion of Britain, it translates German Seelöwe, announced by Hitler July 1940, scrubbed October 1940.
Wiktionary
n. (attributive of sea lion English)
Wikipedia
In heraldry, the term sea-lion (sometimes called a morse) refers to a legendary creature that has the head and upper body of a lion, but with webbed forelimbs and a fish tail. These occur most frequently as supporters, but also occur as crests and occasionally as charges. Sea-lions are frequently found in "sejant" or "sejant-erect" (sitting upright) attitudes, but may also be found "naiant" (horizontally, as if swimming) or "assurgeant" (issuing from the waves of the sea).
Usage examples of "sea-lion".
The Killer whale is an exception, preying on seals, sea-lions and some of the smaller cetaceans - porpoises and dolphins, for example.
As his pace increased so the sea-lions began to bawl out all together - a pitiful exhibition and much too hoarse to be amusing - but as the sudden clamour died away a single disgusted voice, not very loud, came clear across the quiet water: 'Oh the fucking sod.
As the strait grew narrower so the life in it seemed to become more concentrated: the shores on either hand - and not only the little beaches of black sand and pebbles but even the seemingly inaccessible ledges - were crowded with seals, eared seals, sea-lions and sea-bears, lying on their stomachs, on their backs, on their sides, sleeping or making love or merely barking, while others played in the crashing surf or swam by the ship, stretching their necks up and staring with intense curiosity.
There is a sea-lion at the circus, I saw in the paper, who juggles with an umbrella and two oranges.