Doctor Who is known for its wide selection of frightening, unique, and wonderful aliens. In the Classic series, technology wasn't what it is now and the costume department had to think outside the box, leading to some truly amazing creations as well as a fair share of odd aliens.
When the series was brought back as a continuation of the Classic series, the monsters and aliens also returned. Did that mean they were any less unique? Of course not, and it certainly meant the creativeness was back and kicking. From the unique to the uncomfortably weird, the series didn't disappoint viewers.
Vespiform
The Vespiform appeared in the season 4 episode "The Unicorn and the Wasp." In the words of Donna Noble, "When I say giant, [...] flipping enormous [Wasp]!"
A Vespiform can be human but can also change into insectoid form. The Vespiform that starred in this particular episode was murder obsessed and killed in various ways; lead pipe, fallen statue, and stabbing. It was unique and interesting, referencing to Agatha Christie's novel "Death In The Clouds."
Jagrafess
The Jagrafess, also known as The Mighty Jagrafess of the Holy Hadrojassic Maxarodenfoe, made an appearance in the season 1 episode "The Long Game," featuring The Ninth Doctor.
The creature was a mound of flesh with rows of sharp, jagged teeth and a terrible appetite. It required a cold environment as it produced mass amounts of heat. It used screech and growl vocalizations and needed its communication translated by the Editor.
Lady Cassandra
Cassandra refers to herself as the last pure human. The efforts to extend her life resulted in copious surgeries and lead to her final and notable form of a piece of skin with a mouth and eyes stretched across a tall frame.
She would need to be constantly moisturized as her thin skin was prone to drying. Her brain survives as it is immersed in a life-sustaining formula beside her. She is able to transfer her consciousness and reside in another's body.
The Solitract
What shape does the energy that existed before time take when alone? A frog. The Solitract is extremely powerful and complex but incompatible with the universe, exiled to the Solitract Plane. It is able to take any form it wishes.
When the Solitract is alone with The Thirteenth Doctor, it takes the form of a frog and uses the voice of Grace O'Brien, Graham's late wife, to communicate.
Tzim-Sha
And the award for the most uncomfortable face audience members have the displeasure of viewing goes to Tzim-Sha, or as The Thirteenth Doctor likes to call him, Tim Shaw.
Tzim-Sha is of the warrior Stenza and takes teeth from his victims, a brutal way to track and glorify his killings. His collection also means that his face is covered in teeth, as he not only collects them but adds them to his own physical appearance.
Zygons
The Zygons made their first appearance in Classic Who but returned when the series was revived. They are shapeshifting creatures from the planet Zygor, whose bodies and cone-shaped heads are covered in suckers.
They have venom sacks in their tongues, can ooze venomous slime from their bodies, and produce zapping electricity from their hands.
Slitheen
The Slitheen are a family of Raxacoricofallapatorians, aliens from the planet Raxacoricofallapatorius. They can fit themselves into human skin suits and remove themselves from their disguise by undoing a zip on their forehead.
They are extremely gassy and can be defeated by anything of the pickled variety as the acetic acid breaks down their calcium-phosphate based forms.
The Adipose
The Adipose are exactly what their name describes. They are aliens formed from fat, and the tissue that stores energy in the form of fat is called adipose. A human who takes a pill from Adipose Industries does so with the expectation that the pills will cause weight loss.
In fact, the phrase used by the company is "the fat just walks away." Once a person takes a pill, the Adipose form from their fat cells to form a living body. This body can also be formed from bones, muscles, and organs, although this isn't preferred.
Mister Sweet
Mister Sweet is also known as The Crimson Horror. Sweet is a member of the red leech species from the Cretaceous Period and has the appearance of a flayed lobster with vacuous eyes and barbed limbs.
Red leeches attach themselves to living hosts and live symbiotically. They can also produce a poison of fatal potency that has the power to paralyze, blind, and kill.
The Abzorbaloff
The Abzorbaloff featured in the season 2 episode "Love and Monsters." It is a memorable episode as it is one they barely speak of but can't forget. The creation of The Abzorbaloff was not a writer of Doctor Who, but a competition winner.
The end product that starred on the show was correct in essence but different in areas of speech and physical size. The Abzorbaloff absorbed people, retained their memories, while their faces appeared on its body.