The Watcher (Logopolis)

During the moment that the Doctor's fourth incarnation regenerated into his fifth, The Watcher manifested as a physical projection. Eerie in appearance, the Watcher had a white, featureless, unform face and white clothes. He silently appeared observing events on Earth initially and conferred with the Doctor. Later, he transported Nyssa from the doomed Traken to Logopolis to safety.

At the moment of the Fourth Doctor's regeneration, the Watcher merged with the body. (DW: Logopolis)

Another Time Lord, K'anpo Rimpoche, had apparently manifested his future regeneration as his assistant, Cho Je. (DW: Planet of the Spiders)