I probably should've answered a while ago. Here is my take on it:
Bulbapedia put this in Guzzord's bio:
Although [Guzzlord] constantly eats, its droppings have never been found. It is speculated that the entirety of the matter it consumes is converted into energy, with no waste products left over.
And this is from Mudsdale's Bio and Pokedex:
The mud around its legs gives its kicks extra force; one kick can turn a car into a scrap pile. When galloping, its hooves can dig holes in asphalt.
Its heavy, mud-covered kicks are its best means of attack, and it can reduce large trucks to scrap without breaking a sweat.
Guzzlord lacks weight in that it has a large mouth cavity, which is mostly empty space. Guzzlord's food is converted into energy, so it is likely not weighing Guzzlord down. We know, from the Pokemon Company's Sun and Moon Adventure Guide that Guzzlord's tongue (And most likely the rest of it's body) is made of a Rubbery Material. One cubic meter of rubber weighs about 1,522 Kilograms.
Mudsdale, on the other hand, is a horse. Horses are pretty fricken heavy. People who rode horses in war were constantly aware of the threat of their horse falling or dying, because the horse would probably break their legs and be too heavy to remove. Most Horses can weigh from 380 to 550 Kilograms (about half a ton), and those horses are 1.63 meters high. But Mudsdale isn't your ordinary horse. Mudsdale towers over that horse at 2.5 meters high. It can break cars in one kick, which requires serious weight. And Mudsdale has it, in the form of mud and some sort of rocky body. Mud is dirt and water, and water is surprisingly heavy in it's own right. One cubic meter of Sand filled water (slightly lighter than mud) weighs 1,922 Kilograms. Every burgundy-brownish spot on Mudsdale has that weight per cubic meter. Not only that, but Mudsdale's skin is likely made of a more rocky material, like Granite. One cubic meter of granite weighs 2,691 kilograms.
Therefore, Mudsdale, being a massively heavy and Tall horse, would definitely be heavier than Guzzlord, which is likely a lump of rubbery material. :P
Hope I Helped!
Source: A Weight Calculator, Knowledge, HT, Experience, and the clickable links above :P