In the 50 plus years of Doctor Who there have been many many alien species introduced and not all of them were nice. Some bad ones have been The Master, Cybermen, Adipose (cute but don’t let that fool you), Sontarans, Silurians, Zygons, the Silence and the Weeping Angels. I have a Cyberman figure and an Adipose which glows in the dark but under no circumstances will I ever allow a Weeping Angel in my presence. Blink around one of those and you are done for!
I would say that THE monster one thinks of first when Doctor Who is mentioned are the Daleks. Nasty bunch and just as the doctor has changed so have the Daleks which has made them even deadlier. When they first appeared they could only roll about on flat surfaces so making a break for it up stairs meant safety. Then they evolved and learned how to fly. When that first happened with Doctor #9 both he and fans went “Oh &^%$!”
They even come in different colors now based upon their Dalek hierarchy and their purpose. This blue one is listed as a strategist Dalek. Exterminate! Exterminate!