Venusians are a three-eyed alien species.
The Venusians infiltrated Earth for several decades in preparation for an invasion. Part of their infiltration included disguising themselves as humans. Dr. Marvin Monroe was one of the disguised Venusians. When Monroe learned that the Pod People aliens were also invading, he said he was ready for them, although a pair of invading Martians revealed that they too were invading, and prepared to deal with the Venusians.
Behind the Laughter
Their physical appearance and Monroe's unmasking are a reference to the Twilight Zone episode "Will the Real Martian Please Stand Up?", where a three-eyed humanoid Venusian reveals his people have intercepted a Martian invasion so that they can invade themselves.