While watching Mickey's Once Upon a Christmas, I heard the character Pete say the phrase "reindeer what fly" instead of "reindeer that fly". As a native English speaker, I have never used "what" that way before, nor have I heard anyone use it that way in real life. But I have heard it on TV before, so I'm thinking it must just be a regional thing.
Is this usage of "what" found in a particular dialect/region of English? If so, which one?