The Komodo Dragon, a monitor lizard native to Indonesia, is the world's largest species of lizard, known to grow over three metres (ten feet) long. While they can't fly or breathe fire, they have poisonous saliva, and been known to attack and kill humans on rare occasions. Also, they are capable of parthenogenesis (producing young without genetic input from a male of the species), meaning a single female can colonize an island, producing males (thanks to Komodo Dragons following a ZW sex-determination system, unlike humans who follow an XY system, the resulting offspring will always be male), then mate with these males to produce a population. Thankfully, captive Dragons can recognize individual humans and can be trained to a degree. Among reptiles, they are unique for showing explicit affection for the people they really like.
They're non-venomous. Known as a "septic bite" due to their mouths being a host forflesh-eating bacteria that the dragon, itself is immune to, one good bite is all that's needed to infect the prey then patiently wait for it to die then eat at it's own leisure.
The point is arguable. Varanus komodoensis does (supposedly) possess a mild venom, though shock, blood loss and outside infection of the prey's wounds are almost certainly far more important factors than venom might be in the process of prey capture. The protiens inside of it's jaws (used for clotting blood, lowering blood pressure, paralyzing muscles and inducing hypothermia, which leads to shock and loss of conciousness) are also present in other predators; a few of which aren't at all poisonous or venomous, so it's still not entierly certain if the dragons are truly venomous or not.
The Komodo dragon's prehistoric cousin, Megalania grew to nearly 30 feet long.
Bearded Dragons are a variety of related species of lizard native to Australia. Ironically, it is one of the few things in Australianottrying to kill you: they have no special qualities besides a distinctive appearance (with a scaly 'beard' around the head), and are often kept as pets.
Also common on the pet trade (and some of which are also from Australia) are Water Dragons, two species of lizard somewhat smaller than the Green Iguana, and notable for their gentle and mild dispositions (as opposed to the Iguana's tendency towards being a bit more high-strung).
There are also frilled dragons, which have a frill they expand to look bigger, and sailfin dragons, which have a large sail-like crest on their back and tail (or at least, males do). Sailfin dragons are the largest agamids; while still considerably smaller than a Komodo dragon, they're impressive lizards about the size of a Green Iguana.
A genus of tiny lizards which use extendable ribs to support gliding membranes bear the taxonomic designation of "Draco", meaning "dragon". Not exactly a menace to the village, but they're the closest thing there's been to flying dragons since the pterosaurs died out.
They're not named for dragons, but North America's short-horn lizards (aka "horned toads") are about as close as Nature has come to the archetypal limbed-reptile-with-a-Breath Weapon. While they can't actually breathe fire or poison, these spiky lizards can squirt blood from modified tear ducts, that can travel up to three feet and is laced with a compound that's noxious to coyotes, kit foxes, and other canid predators.
Generally averted with pterosaurs, unless your dragons look like this. Prehistoric marine reptiles like plesiosaurs and mosasaurs however pretty much resemble typical marine dragons, and the mammal-like reptilesynapsidDimetrodon looks like a one winged western dragon.
Azhdarchid pterosaurs (whose group name is actually based on a type of dragon) and Ornithocheirid pterosaurs are fairly similar to flying dragons; the former group has the massive size and carnivorous diet of a dragon and the latter has members with sharp, vicious looking teeth and weird head ornaments. The name of one Ornithocheirid, Guidraco even translates to "malicious ghost dragon" (the name's a bit of a misnomer, as despite its ferocious appearance◊, it was most likely a harmless fish-eater).
A number of fossils are advertised in Chinese traditional medicine as "dragon's bones", including prehistoric mammals. A number of species have even been discovered from fossils sitting in Chinese folk medicine shops. One famous example is of Gigantopithecus, the largest known primate in history.
Weedy Sea Dragons, aquatic seahorse relatives that have weed-like growths on them for camouflage in their seaweed habitat. They may not have wings or a Breath Weapon, but they certainly look fantastic.◊
Less fantastic are barbeled dragonfish, which are small, deep-sea fish similar to viperfish. One distinctive feature is that they have several vertebrae missing just behind their skull. This is theorized to be an evolutionary development that permits the dragonfish to pivot its head in ways most bony fish can't.
Dragonflies. They are predatory insects from the day they hatch, although they live in the water until they almost reach adulthood, then they transform into an aerial predator. While current dragonflies are small, having wingspans of a few inches, ancient dragonflies dating from the times of Carboniferous era (about 300 million years ago) can grow to a wingspan of about 70 cm or more, i.e., more than two feet.
The Germans have built a giant walking, fire-breathing robot dragon called Tradinno.