The 40-foot alien monster was actually a marionette about 15 inches high. It was essentially a combination of a rat, bat, spider, and crab.
$54,000 - or just over a quarter of the film's budget - was spent turning the footage into the so-called Cinemagic process.
The "Cinemagic" process, used for all scenes on the surface of Mars, was the result of an attempt by producer Norman Maurer to turn live-action footage directly into hand-drawn animation - or to simulate that. This would enable hand-drawn backgrounds to look as real (or as unreal) as the live action footage. It didn't have that effect here, of course. See The Three Stooges in Orbit (1962) for Maurer's attempt at using his original process he patented in 1955 announced two years later in association with his company, Illustrated Films, called "Artiscope," animation by automation by which live action was photographed and turned into animated sequences without the use of an artist.
The much-touted Cinemagic process which was used for the scenes set on Mars was actually the result of a film-developing mistake. The budget was slashed mid-production so the producers considered turning the film into black and white to keep costs down. However, one reel became accidentally double-exposed which produced a shimmering, vaguely psychedelic glare that director Ib Melchior latched onto, thinking it would suit his purposes for the Mars scenes. (It also helped to camouflage the cheap Martian monsters and scenery.)