How do you get a Sylveon?

While its evolution method mirrors that of other Pokemon games, you will need to make it your Buddy and raise its hearts to 70 in order to evolve it. Niantic / The Pokemon Company (via Reddit: u/hugomagice1017) Use the name trick to evolve Eevee into Sylveon.

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Keeping this in consideration, why is my machop green?

Moreover, does a shiny ditto transform into shinies? In all of the games (excluding spin offs, as I have not played them much) yes, ditto can transform into a shiny pokemon. It's just a sprite, though, and it won't make ditto shiny forever as expected. To add to that, a shiny ditto transforming into a normal pokemon will reveal that pokemon's shiny form as well.

Keeping this in consideration, are shiny feebas rare?

Feebas' shiny form is spawning 1 in 63 spawns. When you combine this a major boost in its spawns in general (previously it was a rare spawn) then it's no wonder trainers feel like the shiny rate is as likely as it is. Feebas isn't the only shiny form that's spawning nearly 2% of the time, however.