one of the many reasons I cannot spend extended time in any branch of the furry community
AKA: dogs do not have 'backwards knees', the PSA.
I have helpfully labeled this picture of a dog with the parts of its leg that are analogous to various parts of the human leg. Notice its knee bends the same way as every other mammal's.
Dogs are 'digitigrade'. Digitigrade means they walk on their digits, i.e. toes (and fingers, in humans and other animals with hands, but we do not call any of dogs' digits fingers) and no other part of their feet. The rest of their foot extends up off the ground. Digitigrade is the default setting for most mammals. Mammals that walk plantigrade walk on the plantar part of the foot, as well as the toes. Humans and many other apes are plantigrade, and there are some animals that can shift between the two based on posture and speed; kangaroos are pretty well-known for this, but cats also adopt a plantigrade posture for many of their positions and many can 'stand' on their hind legs in a plantigrade position, with butt on or raised off the ground depending on how good the cat is at balancing. Japanese breeds like the bobtail tend to be naturals at this, but most other cats can be taught it as a trick if they don't do it themselves. As they can't walk like this, though, they're still considered digitigrade. Also, many humans adopt a digitigrade position when crouching, depending on the steepness of the crouch, although a crouch can also be on the bottom of balls of the feet which isn't quite digitigrade (where the front of the balls of the feet is used as the base).
ETA I suppose I should mention the third option, unguligrade, in which the animal walks on its nails. All ungiligrade mammals are classified as ungulates; hooved animals, basically. So horses, donkeys, giraffes, deer, etc.
Also, these three leg-shapes/walking styles are not just for mammals, but mammals show the most diversity...most reptiles are plantigrade, all birds are digitigrade, etc. But these are the three possible arrangements for all vertebrates that ever walk.
This has been your infodump of the day.
I have helpfully labeled this picture of a dog with the parts of its leg that are analogous to various parts of the human leg. Notice its knee bends the same way as every other mammal's.
Dogs are 'digitigrade'. Digitigrade means they walk on their digits, i.e. toes (and fingers, in humans and other animals with hands, but we do not call any of dogs' digits fingers) and no other part of their feet. The rest of their foot extends up off the ground. Digitigrade is the default setting for most mammals. Mammals that walk plantigrade walk on the plantar part of the foot, as well as the toes. Humans and many other apes are plantigrade, and there are some animals that can shift between the two based on posture and speed; kangaroos are pretty well-known for this, but cats also adopt a plantigrade posture for many of their positions and many can 'stand' on their hind legs in a plantigrade position, with butt on or raised off the ground depending on how good the cat is at balancing. Japanese breeds like the bobtail tend to be naturals at this, but most other cats can be taught it as a trick if they don't do it themselves. As they can't walk like this, though, they're still considered digitigrade. Also, many humans adopt a digitigrade position when crouching, depending on the steepness of the crouch, although a crouch can also be on the bottom of balls of the feet which isn't quite digitigrade (where the front of the balls of the feet is used as the base).
ETA I suppose I should mention the third option, unguligrade, in which the animal walks on its nails. All ungiligrade mammals are classified as ungulates; hooved animals, basically. So horses, donkeys, giraffes, deer, etc.
Also, these three leg-shapes/walking styles are not just for mammals, but mammals show the most diversity...most reptiles are plantigrade, all birds are digitigrade, etc. But these are the three possible arrangements for all vertebrates that ever walk.
This has been your infodump of the day.