In dreams, anything can happen. Flynn is aware of this, and to a degree has accepted this. Even if he hasn't dreamed for years, it was generally common knowledge. Sometimes the mind just dredged up weird things.

Having Mickey Mouse running around in his dreams was where he drew the line. There was no mistaking the shape, the face... was it just from how bright it was? He rubbed his eyes as though that would help clear his vision.

"Am I tripping out..?" he murmured, eyes following the erstwile king. "I haven't dreamed about Disney since I was a kid!"