Passant is the most noble Gesture of a Lion, it expressing his Majesty, Clemency, and Circumspection.

From The Grammar of Heraldry by Samuel Kent, p137

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[The unicorn] is a Representation of Strength and Courage, and also of vertuous Dispositions and Abilities.

From The Grammar of Heraldry by Samuel Kent, p136

The Generous and Lordly Nature of the Lion, will not bear a Rival in the Field; and therefore two Lions cannot be born in one Coat-Armour, but they must be suppos’d to be Lion’s Whelps, call’d therefore Lioncels.

-From The Grammar of Heraldry by Samuel Kent, p51

(An exception is made for any arms belonging to royalty, most notably that of England.)