Cuckoo

Cuckoos was a species that was often compared to alien species that infiltrated other species. (AUDIO:  The Assassination Games, TV: Night Terrors, PROSE: Houdini and The Space Cuckoos, COMIC: Cuckoo) The phrase "cuckoo in the nest" was coined after the process. (PROSE: The Devil Goblins from Neptune)

Examples of Cuckoo-like species
The Cuculus — also known as Space Cuckoos — was a species of humanoid lookalikes of Cuckoos that infiltrated planets and replaced the natives' minds with their own. (PROSE: Houdini and The Space Cuckoos)

The Seventh Doctor once called an extra-terrestrial fossil in the caves around the Lifton coast a "cuckoo [that needed to be pushed] out of the nest of human history", else Charles Darwin's The Origin of the Species might never be written, since the fossil could not be integrated into Darwin's theory of evolution. (COMIC: Cuckoo)

The Light, a species believed to be the secret rulers of the universe, were compared to Cuckoos by Ace. (AUDIO:  The Assassination Games)

Tenza was a species that hatched in the millions that the Doctor compared to Cuckoos, referencing how they infiltrated nests. (TV: Night Terrors)

Vacants were alien Cuckoos. (PROSE: The Pictures of Emptiness)

Behind the scenes
While Cuckoos are often compared to other species in the DWU, there has yet to be any source that adequately defines what a cuckoo is.