At the beginning of the film, The Lord of Horror is believed to be Boogey Man, who planned on stealing and using The Left Hand of Horror to spread fear.
However, after Boogey is defeated, Billy from the Future travels back in time to warn everyone that Mandy will become The Lord of Horror if she gets ahold of The Left Hand of Horror, and when Billy states that she only took over the world in 2 weeks, Mandy states that "sounds about right."
He is then later revealed (as a gag) to be Fred Fredburger, who then decides he does not want to be Lord of Horror anymore and would rather have nachos instead.
WARNING: The following is unofficial speculation, and should not be taken as fact
Since Billybot and Mandroid from the future did not go back in time to help Mandy, but tried to help Boogey instead, it is possible that there were two alternate futures: one in which Boogey retrieved the hand, and one in which Mandy received it.
It's also possible that the gag reveal where it turns out Fred Fredburger was Lord of Horror may have resulted as a result of the time travel.
End of unofficial speculation.