Biography
Some animals are drawn with a tail unlike what it would have in Real Life. Either its tail is shaped differently from what its Real Life counterpart would have, or its tail moves around differently from how it should in Real Life. Sometimes its tail is shaped the way it would be shaped in Real Life, but the shape is more exaggerated than it would really appear.
One example is that some cartoon dogs are drawn with tails that look like that of a cat. Dogs may have the widest variety of tail shapes of all animals, but most dogs in Real Life do not have tails that look completely like a typical cat's tail - long with a round or blunt end. A variant of this trope is for a dog with a tail close to the look of a cat's tail but still typical of the breed to have its tail move like that of a cat at least sometimes. In Real Life, a dog's tail should not curl, twitch, or wave like a cat's tail, even if its tail does look like a cat's tail.
Other common examples:
Wolves with overly bushy tails. In Real Life, wolves have somewhat bushy tails, but not to the extent of foxes.
Cartoon cats from the 1940s and onwards that have tails that taper to a fine point like that of a newborn kitten, albeit longer if an adult cat. Older kittens and adult cats in real life usually have long tails that have a rounded or blunt end. A cat's tail can taper, but usually not like the tail of many cartoon cats. Longhair cartoon cats' tails tend to be drawn like wolves' tails or foxes' tails.
Cartoon chipmunks that have deer-like tails. Real chipmunks have much longer tails similar to that of a squirrel, but thinner.
Speaking of squirrels, cartoon squirrels' tails are usually neater looking than the brush-like tails they have in real life.
Most animals with bushy tails have them portrayed as tapering to a point, when in Real Life, bushy tails are more likely to have a blunt or rounded tip.
Unusually thin and/or fur-covered tails on mice.
Zebras with horse-like tails.
Skunks usually have "neater" tails that taper to a point, rather than the bushy brush-like ones they have in real life. The pattern will often be portrayed with two thin white stripes going through the tail, black at the ends, though typical real life skunks have tails with white at the ends.
Hippos with longer and/or thinner tails than in real life. Sometimes they lack tufts at the tip.
Lions without tufts at the end of their tails.
Jaguars with leopard-like tails. Jaguars have shorter, stockier tails.
Platypuses with fur-less beaver-like tails.
Mandrills with long, baboon-like tails. Mandrills actually have very short, stubby tails, and are no longer classified as baboons.
Monkeys in general with Prehensile Tails, regardless of species. In reality, only certain New World monkeys have prehensile tails.
Then there are extinct animals, where Science Marches On may be to blame for reconstructions that we now know to be inaccurate:
Dinosaurs with overly flexible tails, or ones that drag on the ground. The bigger dinosaurs held their tails rigid and level with their bodies. Even stegosaurids, who were able to swing their tails a full 180 degrees, still held them off the ground.
Saber-toothed cats tend to be depicted with long tails similar to other big cats. This was true of primitive saber-toothed cats like Machairodus, but advanced ones like Smilodon had very stubby tails like a lynx or a bobcat.
Pterosaurs (especially if it's a Pteranodon) with long tails. Most of them, including the Pteranodon, had short tails. Some pterosaurs like Rhamphorhynchus had long tails, but their heads looked very different from the Pteranodon mould (with a long crest) that is most commonly drawn.
Diplodocid sauropods like Apatosaurus and Diplodocus will sometimes be portrayed with rather short tails, despite the family being known for having long, whip-like tails. Conversely, brachiosaurids had relatively short tails compared to other sauropods, but they are sometimes drawn with long tails similar to those of diplodocids.
Mosasaurs were long thought to have paddle-like tails. It is now the consensus that they had a shark-like vertical fluke.
Please only list examples that are clearly based on actual species. Funny Animals clearly based on actual species are allowed too. Aliens, monsters, and mythical beings can have Bizarre Alien Biology, so please do not list them.
Subtrope of Funny Animal Anatomy. See also Furry Ear Dissonance. See Inexplicably Tailless for normally tailed animals that don't even have tails.