Pantsless male Funny Animals: Much of character recognition in animation is based on silhouette. In twentieth century western culture, it was customary for men and boys to wear trousers. The legs of a bipedal critter, except the legs of many bird characters, already have a silhouette not unlike trousers; compare Long Pants. Adding an untucked shirt to the silhouette results in pantsless appearance once the backlight is turned off. Examples include Donald Duck, Wakko Warner, Winnie the Pooh, and Porky Pig.
Shirtless female Funny Animals: In twentieth century western culture, a skirt, whether with underwear or not, was a tertiary sexual characteristic that was used as an easily silhouettable shorthand way to say that a character will fill some approximation of a feminine gender role. They can go shirtless because they lack a certain secondary sexual characteristic. Like with shirtless male animal characters, adding a shirt would push the character too close to the Petting Zoo People tier. Rarely do half-dressed female animals wear pants, shorts, or overalls without a shirt. Examples include Ortensia, Dot Warner, Cindy Bear, and sometimes Minnie Mouse, as well as the 1999 version of Winnie Woodpecker.
Rarity of bare-bottomed female Funny Animals: Again, a skirt is used as a tertiary sexual characteristic. Otherwise, you may need to add a hairdec. Notice Daisy Duck's huge bow. The most common examples bare-bottomed or pantless female animals are birds, but a few are mammals. Dixie Kong and Gosalyn are two other such examples.
Rarity of pantless or bare-bottomed, shoe-wearing Funny Animals: Shoe-wearing pantsless or bare-bottomed cartoon animals as there are shoe-wearing shirtless cartoon animals because being shirtless with shoes on is more accepted than being pantsless with shoes on in the case of humans and most half dressed cartoon animals are barefoot anyway. Daisy Duck and Gosalyn are two female examples and Conker, Daniel Tiger, and Fowlmouth are three male examples. One exception in both real life and cartoons occurs when the shirt is at least calf-length, like that worn by pre-CGI Alvin Seville.
Shirtless, shoe-wearing Funny Animals: Again, they are more common than pantsless characters wearing shoes because being shirtless and wearing shoes is accepted than the pantsless variant. Mickey Mouse and Minnie Mouse are two such examples.
Rarity among Petting Zoo People: Few Petting Zoo People appear half dressed, whether shirtless (like were-form Wilford B. Wolf and Amy) or pantless (like SallyAcorn and Sabrina). Half-dressed Petting Zoo People are less common than half-dressed Civilized Animals and Funny Animals because Petting Zoo People are generally taller (in heads) and look more anthropomorphic than Funny Animals. That means they have room for more clothing. Furthermore, Petting Zoo People dresslike humans as a shorthand way to say that Petting Zoo People think and act like humans. Shirtless female Petting Zoo People are rare because they are anthropomorphic enough for Nipple and Dimed to take effect. Shirtless male Petting Zoo People are more common than the shirtless female and pantsless variants because shirtless men aren't taboo like the other two variants.
Rarity of shirtless normal and Talking Animals: Few Nearly Normal Animals, Talking Animals, and Partially Civilized Animals are shirtless with pants on, let alone fully dressed for that matter, because their body shape is basically as non-anthropomorphic as that of normal animals. Normal animals are almost never given pants to wear because they can't take off clothes easily and they'll likely soil themselves otherwise. Furthermore, animals in Real Life are nearly always naked. Therefore, their cousins on the Sliding Scale of Anthropomorphism wear little or no clothing as a shorthand that they "think and act" like animals.