Fictitious and Symbolic Creatures in Art, by John Vinycomb, [1909], at sacred-texts.com

Sea-lion

or Lion poisson, a mythical sea-creature, frequently used in heraldry as an emblem of bold actions achieved on the ocean in the country's service. It is depicted as the fore part of a lion with webbed feet, the hinder part ending in a fish's tail.

Two such animals support the arms of Viscount Falmouth.

The Earl of Howth has for supporters a sea-lion 

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argent, and a mermaid, proper. The crest also is a sea-lion.

The crest of Duckworth is a tower, the battlements partly demolished, from the top flames issuant proper; on the sinister side a sea-lion erect azure, pressing against the tower.

Silvestre.—Argent, a sea-lion couchant azure, crowned armed and langued gules.

When the sea-lion or other compounded creature of this kind is erect, it should be clearly blazoned as "a sea-lion erect on his tail," to distinguish it from naiant, the swimming position natural to it.