@Background Pony #CB31
It's less about feathers, and more about the cleaning — Preening also can just mean putting on your finest duds and such. And IRL horses are said to be 'preening' when they clean each other's coats with their teeth.
As far as I know, on this site I think it's been exclusively used to refer to Pegasus or Griffons cleaning their wings, but now that we've had a whole day devoted to it some artists have said they're going to go with the Puttin On The Ritz
So, maybe even Trixie could be said to be preening.
I don't know if that means it shouldn't be implied, but I do think yesterday's event broadened the possible meanings of 'preening'.