Primitive but not-aggressive unless provoked, the Ymirs are oviparous green skinned two meters tall reptilian scaly bipeds with a long forked tail and a sagittal crest that extend into a long ridge running from the top heir head to their tail. They have a humanoid torso. Both their arms ans feet are clawed and three-fingered. Their lower legs are turned backward and their face has vaguely simian features, with jowls hanging from their cheaks. They have large nostrils, pointed ears and a pair of eyes surrounded by protective ridges. They have no heart or lungs but a "network of small tubes throughout (their) entire body" and breathes through 'fiberous filtering elements" that protects it from airborne poisons. They breathe sulfur. Under otherworldly conditions, Ymirs are known to be able to grow to much exotic proportions than their native one. On Earth, for instance, they have been known to grow up to 7 meters.
Source: 20 Million Miles to Earth, 1957, Morningside Productions / Columbia Pictures. Directed by Nathan Juran. Based on a story by Ray Harryhausen and Charlotte Knight, put into screenplay by Bob Willaims and Chsritopher Knopft.