Uncanny X-Men, Vol. 1 #126 (1979) 1st Full Proteus

1st Full Proteus
How Sharper Than A Serpent's Tooth...
The X-Men arrive at Muir Island, leaving the Beast behind in their rush to leave. Reunions are brief, as Proteus is on the loose, consuming host bodies almost as quickly as he can find them.
Release Date: 1979