is it only ponies who become anthro we're looking for?

There is a Zebra, Donkey and Mule in the community for starters.

how much of the actual transformation is required to be interesting, so long as said character was not anthro when the story begun, that is.

Skaltrox Defiance Knight and I are working on a collab for his story, Lavender Heart, where a group of friends ends up becoming anthro versions of the Mane 6 (and possibly adding Spike and Starlight Glimmer later on).  Anypony want to help?

I set up my contribution to the google docs as honestashleyjsmith@gmail.com (aka Applejack's email for the story).  Twilight Sparkle also set up her email as well.

We need Rainbow Dash, Rarity, Pinkie Pie, and Fluttershy, and possibly Spike and Starlight Glimmer. 

The characters in this story start as human but end up as the anthro versions of the Mane 6.

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Check out Apple Frost's story on Zecora. Looks like in the Lavender Heart collab, just about everyone who shares a common trait with the pony that they will become will end up getting transformed into that pony (anthro version).