Tailfin Walking

One trope that catches animators when portraying fish or cetacean (whale, dolphin and porpoise) characters in cartoons is to have them walk on their tailfins (or tail flukes if a cetacean) when on land. This makes even less sense than a cow with a prehensile tail - cow tails are relatively rigid, but prehensile tails are viable, whereas tailfins just aren't meant to support an animal's weight - but it's easier to show a fish walking on its tailfins than on its pelvic fins. It makes slightly more sense with pinnipeds (walruses and seals), whose flippers are their back feet, or for merpeople whose tail is shown as homologous to a pair of legs.

This is a subtrope to Feather Fingers.

Examples of Tailfin Walking include:

Anime & Manga

This is an explicit rule of mermaids in One Piece. Around age thirty their tail fin splits in two enabling walking on land (among other things). They're also capable of moving around on their tailfins however.

Commercials

Charlie Tuna from the Star Kist commercials.

Comics

Subverted with Fish Police. While the characters are in the upright, fins-at-bottom position of most humanoid fish characters, they seem to float more than walk.

Film

The fish in Shark Tale sometimes walk on their tailfins, but they stay in the water the whole time.
- This is averted with the shark characters, however.
Averted with the fish in The Little Mermaid except during the "Under the Sea" song, where they walk on their own tailfins underwater.
- House of Mouse normally averts this with Ariel, who either shows up as a human or a mermaid, but plays this straight with her father King Triton.
- Some Disney Princess artwork showing the princesses in a group will for some reason show Ariel as a mermaid, and standing on her tail!
Averted in Madagascar 2: The shark that follows Morty onto the beach, through the jungle and into a volcano doesn't walk, it just lunges forward repeatedly.
The fish character in the Disney live action Alice in Wonderland by Tim Burton.
Willie the Operatic Whale in Make Mine Music, although it was only during his daydreams.
Averted in Finding Nemo.
Fish from Chicken Little.

Literature

Mentioned in The Lorax when the Humming-Fish are sent away:

"They'll walk on their fins and get woefully weary."

Live Action TV

Implied to be done by the "land shark" that showed up in early seasons of Saturday Night Live.

Video Games

Averted in Chrono Cross - mermaids don't walk while on land, they hover. (You can see some "magic sparkles" around Irenes's tail when she's moving.)

Western Animation

Kenny and the other fish characters in Kenny the Shark walk on their tailfins on land.
In one of The Simpsons' Treehouse of Horror episodes, the dolphins who took over the earth walked on their tail flukes when on land.
In the Dr. Blowhole episode of The Penguins of Madagascar, the main villain, Dr Blowhole, stands on his tail flukes and moves around on a scooter.
Jabberjaw, as seen in the page pic.
Misterjaw in the 90-minute The Pink Panther show
Pearl, the whale character from SpongeBob SquarePants.
- Rare pelvic fin variant: The fish characters in the same show.
An old cartoon had a female competitive swimmer win the trophy, we pull back, revealing her to be a mermaid, and she walks away by tip-toeing on her tail fin.
Animalympics has the dolphin Mark Spritz and the orca Ono Nono stand and walk on their flukes.
The water-fearing mermaid in American Dragon: Jake Long was guilty of this on occasion. Averted with the older mermaid who was Jake's principal, who hid her tail by using a wheelchair.
Sometimes played straight and sometimes averted in Chuck Jones' adaptation of The White Seal.
The fish in Fish Hooks all stand and walk this way. And yes, their tails look absolutely nothing like actual fish tails at all!
Narwhal on Almost Naked Animals.
Sally the fish on Scaredy Squirrel plays this straight, although she also averts Fin Fingers.
One episode of Arthur featured an Imagine Spot where Buster Baxter envisions himself as a fish-based supervillain who lives inside a space station shaped like a giant crab, and plans to take over the world by hypnotizing fish so that they do this to "eat land people."

Real Life

This trope occurs in water in a standard training routine in shows with trained dolphins.
Sirenomelia, also called mermaid syndrome, occurs in human beings when the legs fail to separate in the womb. It is linked to kidney problems.