Lizards with the ability to control the sizes of their bodies, making them as small as salamanders or as large as horses. They posess human intelligence and claws dextrous enough to make use of it. Oddly, neither male or female is fertile - they reproduce by transforming other species into their own. The male impregnates the female with his sperm, which infiltrates a single egg. Male and female are trapped together for a day by the males' unusual penis, which extends a long tube deep into the cloaca, rooting itself into the birthing canal and directly inserting semen and other fluids into the womb.
The female then starts to grow 'pregnant', and when she separates, must find another species, two creatures that are likely to mate. Humans are their favorite target. She lays her egg into a convenient valley on the male, usually the navel, underarm or other warm place, while he sleeps. The egg doesn't carry a baby, but instead injects it's genetic code into the males' body. The male awakens soon afterwards sexually insatiable. During the resulting mating with his female, his egg reproduces itself and embeds itself into the female as she rubs her body against him. And finally it injects fluids that trigger the male's transformation into an iguana. If the female doesn't escape by the time he climaxes, his tube extends into her womb and they become inseparable. Immediately, his tube secretes a number of fluids: semen, energy and nutrients from his body for egg development, and liquids that will pump her deeply into heat. Soon afterwards they will be looking for somewhere for the female to lay her eggs… The female herself is transformed either by the baby inside her or (if she escaped without coupling) the egg upon her body.