parasite
A type of organism that is inhabiting a host (other organism)'s body, either by getting inside their body through some means or attached themselves to the host. Parasite usually sustain themselves by using the host's body to gather nutrient, reproduce inside their body, and living there, typically at the host's expense (such as increased vulnerability to disease or even death).
Parasites often comes in a form of a worm, though many other types of parasitic creature exists, with different preference for hosts.
On Danbooru, parasite refers to when one creature has "attached" themselves to the host and is clearly visible within that image, not just the traditional concept of parasites. This can also be applied to posts where a creature is being possessed by other organic beings attached to them.