Well, the quinotaur, legendary ancestor of the Merovingians, is a legendary beast - while Melusine, legendary ancestress of House Lusignan, is a legendary creature, but certainly not a beast. 
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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.
Apologies.
Just grokked that I had the wrong Capitalisation in the thread title.
It has always meant to be Minor Magical Focus
Also, I may have been presuming that becoming a Familiar might change its "special" nature.
ie make it more or less Legendary.
If you absolutely want Legendary creatures to be a subset of "creatures with Might" rather than equate to creatures with Might, my suggestion would be to define a minimum might score than to fall back on an interpretation that relies on what is a legend in the real world (a virtue should be based on mythic europe, and mythic magic, not RL stories), or a reputation score (because of the bard issue). How much might is legendary? I'd have to say that's likely to vary on campaign power level. In some campaigns, that might be Might 20. In others, it might be Might 35, because Might 20 creatures in that game come in packs. You could also limit it to a specific Realm instead.
Well, IMO, "creatures with might" is a MMF, while for exemple "summoning", being a subset of ReVi, would be a mMF.
YMMV, of course 
You make me think that we have, kinda, guidelines for this, in ROP: Magic.
So, for example, we could define it as any being with a might >= that of a companion level character in a legendary campaign.
Another, similar but harder possibility is to use RoP: tI (a lovely book I really need to read again, if only for my personnal enjoyment) with orders of demons having a base might, and specific demons having a bonus due to their rank.
So, use the "demon prince" rule as a guide about "how much might above the average you must have"