X-Men vs. Defender

The original X-Men faced an unexpected enemy when the Puppet Master took mental control of Mimic, who could replicate the heroes' mutant powers. Guarding the Puppet Master's criminal lair was a formidable android called the Defender (X-Men #27).

To prepare for a looming mutant menace, Professor X invited Scarlet Witch and Quicksilver to join the X-Men. They declined. The siblings had previously discussed that possibility but became Avengers instead (Avengers #16). Quicksilver, in particular, wanted to distance himself from any reminder of their early days in the Brotherhood of Evil Mutants.

These panels with the android Defender come from X-Men #27 (Dec. 1966).