In the 1978 series Battlestar Galactica, Cylons are the main enemy. Created by a reptilian race, they wiped out their lizard overlords and now seek to wipe out humanity.
The Imperious Leader is the leader of the Cylon race. He's seen only once, in the first episode "Saga of a Star World".
Lucifer, a Cylon administrator, claims that the Imperious Leader is an IL-series Cylon like himself, with a third brain added. This third brain is designed to mimic a human mind. The Imperious Leader has reptilian-like face and brain-case. He wears a hood resembling steel wool around (but not touching) his brain-case. He wears fabric robes which appear to conceal a humanoid body beneath them. It's entirely possible that Lucifer is either mistaken or lying, and that the Imperious Leader is, in fact, a reptilian.
The voice of the Imperious Leader is identical to that of Count Iblis -- a Seraph, and the demonic being which caused humanity to sin and flee Kobol. (Seraphs are the angelic beings which are at the basis of the Colonial religion.)
The Imperious Leader was voiced initially by Patrick Macnee, and later by Dennis Haysbert.