Mega Gengar, Diglett, Dugtrio, Sandygast and Palossand cannot be affected by the move Telekinesis, but a Pokémon transformed into one of these Pokémon can be affected.
If you look at the source, it shows a link to Transform. Since Illusion and Transform are not very similar (Illusion will still treat Zoroark as though it were a Zoroark, but if a Ditto uses Transform against a Piplup, it will be treated as though it were a Piplup, with the same typing, stats, and moves).
Hope this helps! :)