There were quite a few radiation created monsters in Marvel's early monster comics, including a fairly adorable weed with mind-control powers and a ''scarecrow'' made giant and mobile by nuclear radiation.
Franquin's Idées Noires has one comic about a talk-show debating over the dangers of nuclear energy. A woman calls them to claim that nuclear energy is perfectly harmless since her husband worked in a reactor for 10 years and is fit as a fiddle and that everybody opposing nuclear energy is either a hippy or a communist. Turns out her husband and children are horribly mutated and she sprouted lies just because "There ain't a reason that we should be the only ones affected!"
Them! is probably one of the most well known movies featuring this trope.
Last Days of Planet Earth a.k.a. Prophecies of Nostradamus features a few creatures mutated by radiation in New Guinea. Carnivorous Trees, poisonous leeches, flesh eating flying foxes (big ol' bats), and cancer-ridden human cannibals. That last one got it banned in Japan—where it was made.
No, the "softbodied humans"(severely mutated humans) at the very end got it banned.
The frog-people from Hell Comes To Frogtown are likely this, and they likely also have a few more exotic features, given the infamous "Dance of the Three Snakes" scene.
In The Hideous Sun Demon, Dr. Gilbert McKenna is exposed to radiation which causes him to turn into a monster in sunlight.
The Mutos from Godzilla (2014) are ancient creatures from a time when the world was intensely radioactive, and feed off radioactive material. This presents a problem when they steal a live nuclear warhead and use it in their nest at the heart of San Francisco...
Andre Norton's Star Man's Son had mutant creatures in a post-apocalyptic world.
Metro 2033. Looks like the heavily irradiated, post-nuclear Moscow became a gigantic nest of monsters, mutants and dinosaurs.
More accurately called Tape man... Flukes and tapeworms are both flatworms, but flukes do not have scolexes. Tapeworms do. (But 'tapeman' isn't nearly as cool sounding as 'flukeman'.)
The Daleks in Doctor Who were initially explained in "The Daleks" to be the result of a nuclear bomb detonated by the Thals in the distant past. They had locked themselves in metal travel machines to survive the radiation and over many years mutated into something else (although their experiements later reveal them to be a particularly extreme form of Radiation Immune Mutants - they aren't just unharmed by radiation, they need radiation to survive). Most of this was ignored in later appearances and they were completely Retconnned in "Genesis of the Daleks" in the 1970s.
Monsterpocalypse has those cosy radioactive giant insects that eat you, instead of making you a super hero.
Although it was mainly based on hard science, the After the End game The Morrow Project allowed for radiation-mutated animals that were treated as monsters.
Mutant Future had mutated monsters.
Paranoia had at least one mutated monster: giant intelligent cockroaches in the adventure "Into the Outdoors with Gun and Camera".
d20 Modern's Urban Arcana setting has the Nuclear Toxyderm, a pile of nuclear power plant waste given life.
Promethean: The Created has the Zeka, nuclear Prometheans who have the ultimate in Blessed with Suck. Not only do they suffer Disquiet, they spread fallout wherever they go, meaning they can rarely interact with humans and have a lot of trouble undertaking the Pilgrimage. As a result, most of them go Centimanus, and hoo boy, do they go Centimanus. Two NPC examples are Oleg Wormwood (an Eastern European arms dealer who longs to get his hands on a suitcase nuke and start some real fun) and Tsar Bomba (a hulking brute who seems content to just barge into nuclear power plants, subject the staff to a slow death, and bask in the radiation as the place starts to go critical). And that's not even starting on the Carcinomas...
Zeka can have a Bestowment (innate power) that lets them irradiate corpses to bring them back as zombies. The Irradiation tree of powers also includes the abilities to control insects and then, later on, to mutate those insects into giants (ala Them!). It's also mentioned that the Wastelands created by Zeka tend to include huge, mutated invertebrate lifeforms.
Werewolf: The Apocalypse puts a corrupted form of radiation in the hands of minions of the Wyrm. In fact, nuclear fallout is regarded as somewhat holy by the Black Spiral Dancers (werewolves who fell to the Wyrm); one of their greatest sacred sites is a hive under the Trinity test site.
Well, some do. Unfortunately, their canonicity is vague, so it hasn't been confirmed that Deathclaws are Jackson's Chameleons genetically engineered by first the pre-War US government and then the Master.
Certain Perks can turn you into one. Rad Child and Rad Regeneration makes you regenerate when you have radiation poisoning, and Atomic! makes you stronger and faster when exposed to radiation, on top of giving you a boost to your AP regen.
Used seriously, but knowingly, in Metal Gear Solid 3 to explain why some of the animals are bigger (like the tree frogs) or more aggressive (like the gavials) than they are in real life. Para-Medic even compares it to Godzilla.
There has been investigation into whether nuclear accidents like the Chernobyl disaster have mutated animals over generations. It didn't.
In Real Life, this wouldn't be able to happen. The nuclear poisoning will kill any higher animal or at least give health problems. The children of given animals may have some abnormalities but usually not good.
Scientists discovered that numerous species of birds that lives in the general area of the Chernobyl Exclusion Zone have actually thrived and adapted to the irradiated areas. They began creating more antioxidants that are needed to repel free radicals that would normally age the bird into a premature death.
For the ultimate real-world inversion of this trope, check out what The Other Wiki has to say about Deinococcus radiodurans. It's a microorganism (so an inversion on the size front, as well as the radiation-effects front) capable of surviving radiation doses a thousand times greater than what's required to kill a human being. It's a pain in the ass for nuclear reactor operations since it can survive in places where life really shouldn't (like cooling pipes), and it also causes problems with radiation sterilization to preserve food long-term. Being resistant to a whole pile of other things as well, it's known in the world of microbiology as "Conan the Bacterium".