Originally Posted by Kish
Pantless would mean Roy not panting like a dog. The article of clothing is called "pants," not "pant."
Unfortunately, English does not conform to such simple rules.
Both words appear to be missing from most dictionaries, but I did find "pantless", meaning "without pants", in The New Shorter Oxford English Dictionary
, listed as in use since the late 1800s. There was no entry for "pantsless".
From a descriptivist point of view, the words seem to be used with roughly similar frequencies.