What is the origin of the sea lion as an emblem in many flags, heraldry and coat of arms?

Each command within the U.S. Navy is authorized to display an emblem and motto symbolic of its mission. mission. GRAND RAPIDS' ship's seal, or insigne, is dominated by the figure of a sea lion, a mythical beast having the head, chest, and shoulders of a lion, with webbed claws, and the lower body of a dolphin.
Most of the emblems incorporate this type of mystical beast rather than the actual sealion alive today

Lion: the principal animal charge in English heraldry, an ancient symbol of courage, strength and nobility.
In medieval heraldry lions were always rampant (or combatant for two lions) or, less frequently, passant,including gardant and regardant. These are still the most common {positions;} others include statant, saliant, sejant, couchant, and dormant. Modifications include:
1. Coward: the lion has his tail between his legs.
2. Demi-Lion: the upper half of a lion couped.
3. Double-Headed: the lion has two heads on one body.
4. Leopard: the early heraldic term for a lion passant gardant, as
in the royal arms of England.
5. Lion's Gamb: couped or erased.
6. Lion's Head: couped or erased, may be gardant.
7. Lioncel: a term used when there is more than three lions rampant.
8. Queue Forch%82: the lion has two tails.
9. Sea-Lion: the lion has webbed front feet and its hindquarters
are replaced by a fish tail.
10. Tri-Corporate: the lion has three bodies joined to one head.
11. Winged: the lion has a pair of wings, usually eagle's wings.

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