Pinkheart Slug
The pinkheart slug is a flat pink mollusc the shape of a heart symbol. It protects its soft fleshy underparts beneath a carapace made of a flexible, lustrous, rubber-like natural polymer of a bright pink colour. It preys upon humanoid species, though for some reason the vast majority of victims are male.
After hatching and growing to full size, the slug begins its feeding cycle by latching onto the chest of the host, either by creeping upon them during sleep or by jumping on them, and sealing firmly in place. Once there, it begins to stimulate the mammary glands to provide a source of nourishment - this has the side-effect of inducing considerable breast growth in the host.
With this initial food source secured, the slug begins its second phase of growth: it grows a carapace of its rubbery polymer around the shoulders, torso and abdomen of the host. The carapace covering the shoulders soon expands into a pair of large pod-like structures containing digestive and hormonal organs that connect to a system of veins inside the abdomen carapace. These veins burrow into the host's navel and integrate themselves with the host's digestive system, linking it with the similar organs the slug has grown and combining both creatures' digestive systems. The veins also disseminate hormones into the host's bloodstream, promoting further changes - the host begins to exhibit markedly different behaviour: besides the expected attachment to the slug and resistance to its removal, the host becomes more friendly and affectionate, their mannerisms becoming more overtly feminine and flamboyant. An increased sexual appetite is also observable as well as a narrowing of the waist.
The slug continues to grow over the host's body into phase three. The largest growth forms around the host's pelvis: a large soft mass that covers the buttocks and genitals. This serves to provide sexual stimulation, further influencing the decisions of the host, and additional adaptations to the digestive system - because the host's nutrient-efficient altered diet and biology renders the original waste elimination systems unnecessary, the anal canal is repurposed as an egg-producing chamber. A protective skirt also starts to develop over the pelvis. In addition to these mutations, further changes occur in the host, including the growth of carapace over the neck and limbs, the expansion and reddening of the lips and a change in hair pigmentation to a bright magenta colour.
The fourth and final phase sees the polymer carapace finish growing over all areas of the host's previously exposed anatomy - the extra-thick layer around the feet forces them to balance on the toes supplemented by a supporting strut protruding from the bottom of the heel, and the features of the face are subsumed beneath the smooth carapace, with only the mouth remaining. The mouth itself is prevented from closing as the musculature of the lips causes them to adopt a round shape. The inside of the mouth adopts the same shape as the teeth degenerate and recede into the ridged internal surface. As the host's original sensory organs are sealed underneath the polymer, the slug's own apparatus take over. The pelvic skirt grows considerably, exceeding the shoulders in width and hanging down to about mid-thigh. The behaviour of the host is significantly altered by this point: the host no longer exhibits their original personality, instead seeking out future hosts to subdue. This is accomplished by dosing the prospective victim with an aphrodisiac serum secreted by the saliva glands, administered to victims orally. The serum confuses victims and leaves them highly suggestive and amenable to slug implantation. Additional does of serum are used to keep developing hosts docile, along with oral stimulation of the genitals.