Not to be indelicate, but you guys know what the nature of a Patronus is, right? It is what the Good Guys use against Dark creatures. Its very nature verifies the caster is not a follower of the Dark Arts. Umbridge was able to conjure one because she was only a Supremist, not an outright follower of the Dark Lord. Lucius Malfoy is a Death Eater, and as such is incapable of producing one.
J.K. Rowling has stated that no Death Eater except Severus Snape could conjure a Patronus. This is because Severus is not a true Death Eater, and because his love for Lily Evans is far stronger than his love for the Dark Arts.
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However...if Lucius were a decent human being, in my opinion his Patronus would be a peacock. Goes with his showy personality. ;)