Note: Please do not tag this page with every retroactive appearance of the Space Phantoms over the 40 years of Avengers comics as these retcons are considered "behind-the-scenes" appearances.
The Space Phantom was introduced in The Avengers #2 and while his home planet of Phantus and race were a plot point in Thor #282, they remained unseen. Despite this, they received an entry in The Official Handbook to the Marvel Universe but it wasn't until What If? #39 that we finally saw the race as they appeared battling the Time-Plucked Titans.
But, none of this matters, as all of it was retconned away in Avengers Forever. In that series, writer Kurt Busiek revealed the Phantoms were at the heart of a conspiracy involving the Avengers and many of their storylines. The Space Phantoms were primarily used as a means for Busiek to retcon over 40 years of stories, often quite heavily, to excuse certain characterization or revelations he simply didn't like(!). The planet of Phantus and previous Space Phantom appearances were written of as part of a convoluted "smoke and mirrors" campaign by Immortus.
Although the Space Phantom has been seen since, no other Phantom has reappeared since Avengers Forever.