Any Pokemon which has access to Mimic, Copycat, Metronome, Transform or Assist in Gen 4 has access to the Mimic glitch, meaning it can learn any move in the game. The glitch only takes place in Gen 4, but a Pokemon with one of these moves can be transferred from Gen 3.
There seems to be a glitch regarding this. You cannot use Deoxys with the glitch. I tried all 4 forms. The same answer every time is that Deoxys cannot learn this move. It learns Mimic though. In Gen 3. I tried Gen 1 moves, Gen 2 moves, Gen 3 moves, and Gen 4 moves. The result was the same each time.
Am I missing something? Does Deoxys' status as an event pokemon have anything to do with it?