I found this great, free Crochet Pony Tail Beanie pattern here at Stitch & Hustle. My sister requested a beanie for a pony tail – not a messy bun – and I love that this one has buttons on the brim so the fit can be adjusted as needed. Also, the opening is large enough so you can put your pony tail wherever you want it.
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The pattern works up quickly and is easy to follow. You could even make it striped as well as a solid color. You could also use fun buttons to dress it up.