Daleks
The Daleks
Terry Nation wrote them quickly and for money; Raymond Cusick designed them to conceal the operator and to move like nothing with legs. Inside is the mutated remnant of a Kaled, a species that fought a neutronic war to its own end and then armoured what was left. They were meant to be a one-off. They came back within a year and have never really left.
The single most important creative decision in the programme's history was made in defiance of its own commissioning brief.