Wild ZUN is a tag that describe any drawn artworks that emulate the art style of ZUN accurately. Any drawn Touhou characters that looks like ZUN's drawings will be tagged under this category.
Aside from drawings, any composed music or game programming that emulated ZUN's style can also be attached using this tag. However, in pixiv, since majority of such drawings are tagged using ZUN drawing, cases where artworks that are tagged under "Wild ZUN" are uncommon.