There is a tendency in fiction to treat all groups of "cold-blooded" creatures as interchangeable and more or less the same thing. This often ends up with the distinct lineages of reptiles, amphibians and fish mixed into one strange package; generally a finned, reptilian or amphibian creature with both lungs and gills. A Sea Monster can have this kind of appearance, as many blend characteristics of fish and reptiles. Alternately, this trope can occur in a single species where some members are Lizard Folk, some are Fish People, and others appear to be Frog Men or salamander-like.
Note that there are some animals out there that are classified as fish, but have amphibian and reptilian features (see the Real Life section). This is partly because the classic fish-amphibian-reptile classification is quite outdated, all three taxa are paraphyletic (i.e. do not contain all descendants of the same common ancestor), thus some animals, traditionally classified as "fish", are closer to amphibians and reptiles on the evolutionary tree. Similar features in different vertebrate taxa can also appear due to convergent evolution. In fiction, this often plays out as if someone took an existing animal and stuck traits belonging to another class onto it.
Subtrope of Mix-and-Match Critters and Artistic License – Biology. See also Call a Pegasus a "Hippogriff", for similar confusion of mythological creatures in general, and Somewhere, a Herpetologist Is Crying, for other inaccuracies regarding cold-blooded animals in media. Has nothing to do with Cold-Blooded Torture.
- The Phantom Menace: The Colo Claw Fish, a Sea Monster living in the oceans of Naboo, has crocodile-like jaws combined with an eel-like body and bioluminescence. Then again, it is an alien. . .
- The kaiju in Pacific Rim all look vaguely reptilian with their scaly skin and clawed limbs. However, two individual ones have very shark-like heads, specifically the Knifehead, which has the jaws and elongated snout of a goblin shark, and the slattern, which has the distinct head shape of a hammerhead shark.
- Harry Potter: Plimpies are small "fish", yet they look a lot like bipedal frogs.
- Aztec Mythology: Cipactli is a mixture of a crocodile, a fish and a toad.
- Classical Mythology: Dragons are reptile-amphibians with mammalian teeth and bat wings, and some have gills. Sea serpents are also fish-snake hybrids.
- Chinese Mythology: Longs are often seen as fish-scaled snakes with legs. Certain naga depictions also have traits like fish.
- Egyptian Mythology is weird because it draws a clear distinction between neutral/good crocodiles and evil water snakes, but turtles are frequently conflated with the latter.
- Hindu Mythology: The makara is commonly depicted as a crocodile with a fish's tail.
- Aboriginal Australian Myths: Taniwha is a reptile of sorts, that lives in and breathes the water.
- In The Elder Scrolls series, the Argonians are a "Beast Race" of swamp-dwelling Lizard Folk. Their physiology is chiefly and visibly reptilian, however they also have elements of amphibians (said to be "sequential hermaphrodites", meaning they can switch genders, though this hasn't been brought up in-game other than from a dubious source) and fish (gills on their necks which allow them to breath underwater, and shows up in-game as a Water Breathing racial ability).
- The Magikarp/Gyarados family in Pokémon can be considered this since Magikarp is a fish that evolves into a sea serpent. As of Generation VI, Gyarados gets a Mega evolution that looks more like a fish, having a huge dorsal fin and a bigger body.
- The Warcraft franchise features the Murlocs, who are semi-intelligent creatures halfway between Fish People and Frog Men, with piranha-like heads and fins on their back, but frog-like limbs and an amphibious lifestyle.
- Some of the Mix-and-Match Critters that inhabit the world of Avatar: The Last Airbender qualify for this trope. This includes the catgator (an alligator-like beast with catfish whiskers, living in the Foggy Swamp) and the eponymous serpent of Serpent's Pass (snake-like body with a seahorse-head).
- My Little Pony: Friendship is Magic:
- Cipactli as a crocodile-faced toad.
- Hydras are also shown to not need air, despite the reptilian-ness.
- Coelacanths and lungfishes are two groups of fish that have some amphibian features, such as two pairs of lobe-like fins that resemble limbs, and in the latter group, lungs. Molecular studies showed that they are more closely related to Tetrapods (i.e. land vertebrates, including amphibians and reptiles) than to ray-finned fish.
- Many fossil tetrapod ancestors, such as Eusthenopteron, Panderichthys, and Tiktaalik, could probably be best described as this, since they were not exactly "fish" in the traditional sense but were not members of the modern amphibian group.
- Ichthyosaurs were prehistoric reptiles that evolved many fish-like (particularly shark-like) features due to convergent evolution, including a vertical tail fin as well as a dorsal fin. Tellingly, their scientific name means "fish-lizard" in Greek. They still had lungs and had to breathe air, and gave birth to live young, similarly to modern dolphins. They probably didn't fit the "cold-blooded" part, though, since they were most likely endothermic.
- The common names of animals often confuse the issue, as with "horned toads" or "sandfish" that are types of lizard.