The imagination of Doctor Who‘s monster is astonishing, but despite introducing some outstanding villains, there are others that never made it past their first on-screen appearance. In the more than sixty years that Doctor Who has been on TV, the few constants remain the Doctor, his TARDIS, and his companions. Meanwhile, monsters and villains make occasional appearances, but the show has fallen into a routine of having a small number of rotating monsters, with many other aliens and life forms appearing in one off stories.

https://screenrant.com/doctor-who-monsters-deserved-more-one-appearance/