The Skeleton Men resemblance to one-eye skeleton aliens, In the second live-action film, They have different color eyes; one red and the other green.
The Scooby-Doo! Show
In the episode, The Three Skeleton Men were the disguises of Dr. Grimsley and his two henchmen. They were hijacking Weather Eye planes and trying to sell them overseas.
Scooby-Doo! 2 Monsters Unleashed
In the second film there are only two Skeleton Men, The Skeleton Men were first seen in the Coolsonian Criminology Museum as costumes, They were later stolen from the museum and taken to the Monster Hive to be turn into real monsters off-screen.
The Skeleton Man was first seen in-counted with Shaggy and Scooby, but Shaggy and Scooby escape the Skeleton Man.
Both of the two Skeleton Men appear a the same time and went after Shaggy and Scooby.
Where the Skeleton Men were finish putting themselves back together, Velma throw a rock at them, Velma run away and the Skeleton Men went after her, Velma went into a pile and escape the Skeleton Men, The lid on the pile closed and the two Skeleton Men jump on the pile and cause the pile to slap and landed on the two Skeleton Men crunching them.
The Skeleton Men return to the Monster Hive as a simple one two-headed Skeleton Man, They join up with the Zombie, The Miner 49er, and the Tar Monster. The Skeleton Men along with the Zombie scream when Fred charge at them, but Fred was stop by the Tar Monster, The Evil Figure tell the Skeleton Men, the Zombie, and Miner 49er to stop Scooby-Doo, Scooby jump on the Skeleton Men's head and Scooby jump off and they bump them heads together. When Scooby-Doo put the control panel back on the machine and turn the machine on, The Skeleton Men started dying and they were turn back into two separate costumes, It is unknown if they were taken back to the Coolsonian Criminology Museum since they were never seen in the ending credits.