Well it's been bugging me all night... the lion and the unicorn, the lion and the unicorn.... where have I heard this before???
Of course... three things, it is a children's nursery rhyme (had to drag out a very old copy of Mother Goose)
The lion and the unicorn were fighting for the crown
The lion beat the unicorn all around the town.
Some gave them white bread, and some gave them brown;
Some gave them plum cake and drummed them out of town.
Based on this bit of history:
King James VI took over the crown from a childless Queen Elizabeth I in 1603, uniting Scotland and England and the royal coat of arms was changed to represent a unicorn for Scotland and a lion for England.
"This was a potent bit of symbolism, for both the lion and the unicorn had long been thought to be deadly enemies: both regarded as king of the beasts, the unicorn rules through harmony while the lion rules through might, It came to symbolise a reconciliation between the Scottish unicorn and the English lion that the two should share the rule."
And also a chapter in Lewis Carroll's "Through the Looking Glass"
Now that makes some sense to me!
I don't necessarily see Strength as innocent but pure, yes, I don't think you have to be innocent in a virtuous way to be pure of intent. The shadow, or beast within, is no longer a shadow if you've confronted it and are no longer afraid of it.
I think that lion and the unicorn in this card could actually represent that fight.