5. Invisible Men
The Whoniverse has had its fair share of invisible characters (not all men, may I add) and they obviously share that trait with Universal Pictures' 1933 classic character; "The Invisible Man". The footprint you see in the picture above is that of one of the Visians of Mira. They appeared (or didn't, so to speak) in the First Doctor's adventure "The Daleks' Master Plan" in season 3 of the original show and are eight-foot tall, vicious aliens who are completely invisible. They attacked a group of Daleks who had come to their planet in pursuit of the Doctor. In the same season, the First Doctor encountered the Refusians - the original natives of the planet Refusis II - in "The Ark". Once possessing a physical form, the Refusians lost their bodies thanks to a giant solar flare and, as a result, they no longer had a recognisable visible "being". They not only becamse invisible to other races, but also to each other and they could only recognise another Refusian's presence by sensing it. In "Planet of the Daleks", the Third Doctor encounters the Spiridons of the planet Spiridon - humanoid natives who used anti-reflecting light waves which made them naturally invisible at all times. This ability made them a target for the Daleks, who wished to duplicate it. However, when the Daleks did so, it worked, but it also completely sapped their power, rendering them useless. "The Face of Evil" - a Fourth Doctor adventure - saw the Doctor on an unnamed planet where he discovered the remnants of an experiment that had been carried out by the deranged supercomputer Xoanon. The experiment had included unleashing invisible monsters in to the planet's jungle, where they would attack the Sevateem tribe. Most recently, when the Eleventh Doctor met Vincent van Gogh in 1890s Auvers-sur-Oise in France during the events of "Vincent and the Doctor", Vincent van Gogh was able to see an invisible Krafayis when nobody else could. The blind, frightened creature attacked whilst the Doctor was trying to help it, and van Gogh accidentally mortally wounded it. The Doctor caressed and comforted it as it slowly faded away and died.