GEESE

Barks' universe contains two geese as main characters - Gladstone Gander and Gus Goose. According to their names they are of the same basic species, but they have very different anatomies in some points:
Head: Gladstone's head is circular, while Gus' is pear-shaped - Hair: Gladstone has a scalp full of curly hair - Beak: Gladstone's beak is flatter and longer (similar to the ducks' but without their rim at the end), while Gus' is narrower, taller and more rounded at its root - Arms: Gus has shorter arms.

In some of the animated shorts Gus has a fanlike bunch of tail feathers sticking out, but Barks always drew him with the standard foul tail in the stories.

CHICKEN

To many readers Gyro Gearloose is very difficult to classify and he is frequently taken to be many kinds of different birds. But it is quite simple, really; Gyro is a chicken. Why? Because Barks has said so himself! And Gyro does perhaps resemble a broiler with his tall, rather slim appearance and the distinct chicken beak.
Although Gyro is dressed as a man Barks did divulge most of his anatomy on a few occasions: He is white-skinned with no sign of feathers, and he has human-like feet.
There can be no doubt that Gyro has hair of some sort, but it is hard to tell if it is real: In U$15 'The Cat Translator' Gyro exclaims: Well, float my toupee!, suggesting that he is wearing a wig. Still, in CP8 Grandma's Present, he seems to have his own hair, because he pulls it vigorously in despair without it showing signs of coming off.
Gyro is the only non-duck main character who wears glasses, but we are never told why he wears them and, consequently, what his eyesight is like.

DOGS

The most famous dogs in Barks' universe are, of course, the Beagle Boys; a group of identical looking characters who do not really look that much like beagle hounds. As dog characters they lack any trace of hair apart from seemingly day-old stubble filling their faces. This is also the case with Goofy - whom Barks drew in FC0079 The Riddle of the Red Hat - and he has even longer stubble but in a smaller quantity.
Barks invented another humanlike and two-legged character who resembles the Beagle Boys namely Donald Duck's antagonist neighbour J. Jones.

Barks also drew dumb dogs that had the physics and appearance of real dogs - the St. Bernard Bornworthy (or Bolivar) and the bloodhound General Snozzie. Anatomical differences: Bornworthy is covered in fur while Snozzie's skin is smooth, Snozzie is very agile and flexible which Bornworthy is not, and Snozzie is able to convey graphic messages using facial expressions and his paws. Furthermore, Snozzie is a tracking dog without parallel; he can track up in the air and under water!

CAT

Black Pete is the only recurring cat character in Barks' stories. It is a mystery why Barks drew so few humanlike cat characters in his stories, because cats have a very facetted overall appearance that must have appealed to him; they seem more agile, more expressive, and more cunning.
Pete premiered in the animated shorts where one of his features was a pegleg (in fact, he was frequently called Pegleg Pete at the time), but the artificial limb was later discarded, because the animators always forgot which leg was the wooden one!
Pete has some of the basic features that the mice have namely black skin and a large, white facial mask. He also has stubble, and in one story (FC0062 Frozen Gold) Barks even equipped him with a groomed moustache. Being the cat character he is, Pete also has pointy ears (where i.e. dog characters always have rounded ears).

MICE

Barks never liked the members of the Mouse family as characters, and he only drew them twice because he was asked to (Mickey and Minnie in FC0079 The Riddle of the Red Hat, and Morty and Ferdie in U$13 'The Lightening Power Box').
Barks' dissociation from the characters was understandable, because the Mouses are very different from his Duck family, where all the characters have rubber-like anatomies that can be used in numerous postures. Furthermore, their faces are very expressive which is not the case with the mice with their small, expressionless eyes. Their overall appearances are also much stiffer in all their expressions and movements.
The mice are hairless and black-skinned except for a white mask that covers most of their faces. They also have thin tails that serve no purpose at all. The ears are in a league by themselves; they are built as two round balls and, apparently, they are tirelessly floating around all over their heads in such a way that the reader always sees them beside each other no matter which position the characters assume in a given panel.

Barks also drew one story in which some much more mouse-like characters (hair, whiskers, and flat ears) participated; it was Gus and Jaq in WDCS132 'Attic Antics'.

METAL STICK MAN

By far the most strange main character in Barks' universe is a metal stick man with a light bulb for head! We never learn how he came about, but it is plausible that Gyro invented him (which also implies that he is able to create at least some sort of life!).
The little character is mostly referred to as Little Helper, although he has no official name, and he is built from metal parts. This was plain to see in U$20 'Roscoe the Robot' in which he was able to, literally, straighten himself out with a hammer after a mishap. This action also implies that he has no physical feelings (as for mental feelings we have once witnessed him daydreaming about a metal stick girl in U$26 Krankenstein Gyro).
Little Helper has a very versatile head - made from a light bulb that he even changes from time to time when it gets broken! Still, it contains both his eyesight, his hearing abilities, and an impressive brain...