... The amount of skin you get is dependent on the size of the animal, but due to the way that the game calculates things, hoofed animals give significantly less skin than their size would indicate, just like they give less meat etc. That'll cause a few problems with balancing things out. If I remember, dogs gave 12 and horses gave only 14...
To skip my dissertation, scroll to the bottom of this post.This is pure conjecture, but I have a theory as to why this phenomenon occurs; fat and meat output from animals, and discrepancies thereof between hoofed and toed phenotypes.
In a sweeping and unfounded assertion, I will state that most of us non-Toadies associate variable yields from butchering (such as meat and fat output) as dependent purely on animal size, wherein the larger the animal, the greater the amount of delicious meats and useful fats. However it is also known that this trend does not follow a normal linear scale with size, and that different body part products begin to be obtained at different sizes (such as hides, bones, meat, and specific organ meats).
I am certain, to certain degree of certainty, that the body size of the creature
in relation to the relative size [REL_SIZE] of the body part and/or organ (as specified in the body_plan), determines if a butchering product is obtained. In the case of butcherable tissue layers, such as muscle or fat, a product may be obtained from each body part if the rel_size of the body part in relation to the body_size of the creature meets or exceeds some unknown criterion. While !!SCIENCE!! is required to elucidate both the validity of this theorem and the critical criteria for successful butchering of organs, meat, and skin (and ect.), this underlying principle leads to my logical supposition on the variance between expected and apparent output of meats from hoofed creatures.
In the body plan of the creature raws, non-hoofed quadrupeds gain the advantage of more body parts than hoofed creatures. Specifically, our unfortunate ungulates lack the digiti pedis... Toes. While the REL_SIZE of the standard quadruped toe is extremely small in relation to, say, the upper or lower body, it is possible the critical body_size to rel_size equation is satisfied once a certain body_size lower limit is reached, allowing our dwarves to harvest the succulent toe-meats of our beloved farm friends.
If this is, in fact, the truth and not an erroneous assumption, then the same body part harvest mechanism could explain the difference in skin glob yield between hoofed and toed creatures after application of your mod. Your dwarves get a bunch of extra toe skin from the dogs, toe skin that evolution did not deign to gift upon the horse.
Short VersionYou get less meat (and in your case skin) from hoofed animals because they don't have toes to butcher. I think.