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; this is especially common for Sea Serpents, which even at their most reptilian often sport rayed fish fins. Alternately, this trope can occur in a single species where some members are Lizard Folk, some are Fish People, others appear to be Frog Men or salamander-like and so on.
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 This is partly because the classic fish-amphibian-reptile classification is quite outdated and all three taxa are paraphyletic (i.e. do not contain all descendants of the same common ancestor), thus some animals, traditionally classified as "fish",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.
- Naruto: The Three-Tailed Beast looks primarily like a turtle, but also has spikey chitinous armor like a crab and its tails resemble that of a shrimp.
- Avengers: Back to Basics: In the second arc, frog Thor is repeatedly described as a lizard. This is subverted when the Maestro irritatedly points out the difference when the Magus makes this mistake.
"And second, he's not a lizard, he's an amphibian. God, I hate stupid people."
- Ice Age: The Meltdown features two Sea Monsters named Cretaceous and Maelstrom, both having a combination of reptilian and piscine features. Cretaceous in particular looks like a crocodile with fish-like fins on his back and tail. They are identified by Word of God as an ichthyosaur and a pliosaur, although they don't really look like their respective species.
- The Monsterverse incarnation of Godzilla, instead of being a dinosaur, is re-imagined as an aquatic Permian reptile that somehow possesses both lungs and gills.
- Pacific Rim : The kaiju mostly look vaguely reptilian with their scaly skin and clawed limbs. However, two individual ones have very shark-like heads, specifically Knifehead, which has the jaws and elongated snout of a goblin shark, and Slattern, which has the distinct head shape of a hammerhead shark. Justified because they were constructed rather than bred, and incorporate whatever features their makers thought would be useful for their roles.
- The Shape of Water's male lead is a strange Creature from the Black Lagoon-esque fish person who possesses both lungs and gills. He's listed as "Amphibian Man" in the end credits.
- Star Wars:
- Cthulhu Mythos: The various eldritch beings sometimes blend the traits of fish, amphibians, and reptiles into weird aquatic monstrous humanoids, such as The Shadow Over Innsmouth's Deep Ones (described as "fish-frog things") or the soft-skinned, amphibious folk of Ib and their god Bokrug the Great Water Lizard in The Doom That Came To Sarnath. Adaptations and appearances in other media will frequently lean even further into this trope by slapping some tentacles on there, too - in the popular consciousness, nothing is really Lovecraftian unless it has tentacles.
- 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 (or "Chinese dragons") 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.
- Pacific Mythology: Taniwha is a reptile of sorts, that lives in and breathes the water.
- Dungeons & Dragons and its derivative Pathfinder dabble in this with a few of the more Lovecraftian demon lords and Great Old Ones. Dagon is usually portrayed as a mix of eel, shark, and mollusc, and Pathfinder features Bokrug the Great Water Lizard with a beard of tentacles. There's also the troglodyte god Zevgavizeb, who looks like a mix of a dragon and a tentacled worm.
- The Lizardmen are a species of Lizard Folk divided into a Fantastic Caste System: three of these breeds are based on lizards first and foremost, but their caste of mages and priests, the Slann, resemble giant toads more than anything else. Further, all castes begin their life in the spawning pools as tadpoles before maturing into their reptilian adult selves.
- Warhammer Fantasy Roleplay: Lakemen are a variant of Beastmen that resemble misshapen humanoid frogs, but also possess gills and crustacean pincers. Justified, as they're creatures of Chaos and Chaos habitually causes living things to develop drastic mutations with no regard for taxonomy or sense.
- Rikuo of Darkstalkers game combines traits of fishes, amphibians, and even mollusks into his moveset, though his mollusk traits are only apparent during certain attack animations.
- The Elder Scrolls: 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 Legend of Zelda: The Zora are mostly portrayed as Fish People, but Majora's Mask reveals that they are born as limbless tadpoles, which is a trait more in line with amphibians, while Breath of the Wild depicts them with sharp claws on their hands and feet, which is more in line with reptiles.
- Pokémon: The Magikarp/Gyarados family 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.
- Total War: Warhammer continues Warhammer's use of this in regards to the Lizardmen, a nominally reptilian race whose castes include the vaguely dinosaurian Saurus, the toad-like Slann, the highly crocodilian Kroxigor and the, well, skink-like Skinks.
- Warcraft: The Murlocs 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.
- Amphibia: Some of the reptiles possess fish-like features. Notable examples include the water snake in "Best Fronds" and the garbage lizard in "Lily Pad Thai".
- Avatar: The Last Airbender: Some of the Mix-and-Match Critters that inhabit the world qualify for this trope. These include 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).
- 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.
- Several species of salamander (most notably the axolotl) retain functional gills into adulthood, and thus live more like fishes than typical tetrapods.
- Caecilians are limbless amphibians which are sometimes confused with snakes or, taking the trope even further afield, for very large earthworms.
- In medieval monasteries, turtles were considered to be fish for the purposes of not eating meat on Fridays and Lent.
- Frogfishes are a type of anglerfish so named because, independently of tetrapods, they have developed four fins modified for walking along the sea bottom, giving them a vague resemblance to toads or frogs.
- In earlier editions of Carl Linnaeus's taxonomy, he wrote about amphibians, but he used that term to mean both amphibians and reptiles, and even some fish like sharks that have cartilaginous skeletons.