Okay, so back to square one.
A mMF needs to be under 1/5 of a form.
Creature = living being.
Magic things are probably out, but everything else is probably in. That's broad. If we consider it to be "Living beings with might", that's almost all of the "Supernatural Beings" MMF I discussed above.
So, this needs further trimming down, and that's where legendary comes in.
Legendary = ???
Thinking about it, I see 2 ways we can define it:
- (1) A relative way, like "Creatures legends are told about"
- (2) A modern way, like "creatures that we know only existed in legends".
Okay, I see a third, similar way: (3) "Creatures that only exist in legends".
Going with (1): Is it narrow enough to be a minor focus, especially given the fact that you can just make anything a legendary creature by taking the time to spread story about it?
Sigh... I dunno. And this is very much saga dependant.
Note, however, that chances are that such creatures will be probably be more abundant in a Saga than in the game world (people seeking the aid of wizards against a well-known monster, wizards hearing about a famous beast and wanting to hunt it, whatever).
If we take Legendary in the (2) somewhat modern sense, we kinda fall back on the definition first outlined by ErikT.
Is it enough to make the focus minor?
That's a tough one. It is broader than my definition for "Beasts of Legends", as in includes spirits, angels, demons, giants, centaurs, dryads... No, better, simpler: This probably makes the focus into "Supernatural Beings save Things, Animals with might and maybe magic humans".
So, IMO, too broad. YMMV
Hey! An idea. Going back to ErikT's idea that these creatures must be "something special"...
Legendary could then mean it must either be one of a kind, or an exceptional specimen of it (For example, if the average might for its kind is X, it is ancient and has might X+10)
This fits Cerberus, Melusine, but excludes a lot of "ordinary" dragons. I don't know. Someone, ideas?
An aside for (3): That's a fun one. It only applies to creatures that don't exist in the game world, so, practically, creatures created by the caster, or faeries that, hearing such stories, decide to take the role of that creature.