This is one of my favourite Pokemon! Along with Zoroark, Espeon, and Sylveon !!! I love foxes and my main fursona is a fennec fox, so I was SOOO excited when they released these images of the starters! I love how you did the colours here - the purple outlines complement the yellow very well.
It's actually just the pen tool in SAI but with a "Spread" texture applied to it, so it gives it that sort of uneven ink feel! Then I set it to Fringe, and color it slightly darker in certain areas to make it pop.