Even if "fairy filly (filly)" won't sound vague (which it does), "rose (filly)" would definitely cause mass confusion. Both can be interpreted as alternate tags for younger versions of various MLP characters.
As for how the franchise is titled:
The Filly franchise is kind of convoluted when it comes to naming (still can't fully understand it). It has an oddity in that, from what I can tell, it is usually named only by section ("Filly Ballerina", "Filly Wedding", "Filly Princess", "Filly Butterfly", "Filly Unicorn", "Filly Elves", etc; with the most well-known obviously being "Filly Funtasia"). And then there's this piece of information I have recently found early 2020 on the franchise's wiki:
I'm not sure on whether or not the name "Filly World" can be used for any tags, as it's likely an unofficial name (even though it apparently did appear on one set). It's also the name of the world within the franchise (Funtasia being a continent in it).
Filly World is the technical term used by various toy makers, sellers and advertisers, for when a playset, surprise egg, puzzle or other types of merchandise that involves the Filly toys, has only been branded merely as Filly. It also often indicates that the toyset in question borrows from multiple of Filly's normally strict toy lines, but cases of one-line sets being branded as Filly has also occured.
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