You can't get a completely smooth perfectly metallic finish with acrylic paint, since it's a suspension of mica fragments in the paint, but you can get good results . . . miniature paints can be really good for this; some are acrylic and some are enamel.
I really, really love DecoArt Metallics in Champagne Gold or Shimmering Silver. Champagne Gold despite the name is actually way more silver than gold, it's just a warm silver, not a cold silver. And it, specifically, is the brightest silver paint I have found that has not been discontinued. I use it all the time, and I mix a little of it with other light or cold metallics like silver or steel if I feel they aren't popping enough.
There's also real foil leaf, which comes in silver, but it really, really depends on what you are going to do with it how well that would work. I've never used it on a pony.
And there's Liquid Leaf which is pretty darn shiny, but it's not acrylic, it's xylene/toluene based, and I have literally NO idea how it would react with acrylics under it or over it, or over raw vinyl. Also, xylene and toluene are really gross chemicals.