Quetzalcoatlus was perhaps one of the last and largest Pterosaurs ever to have lived, with a 10-11 metre wingspan, yet a hollow-boned lightweight body of only around 120-150 kilograms (comparatively very light for it's dimensions and sheer size)
It is an Azhdarchid Pterosaur (the Azhdarchids of the Pterodactyloid Pterosaurs, being the largest known among all the observed families within the Pterosauria)
It lived in the Late Cretaceous of North America, from the Maastrichtian Stage, over 66 million years ago. It was named after the Meso-American Aztec feathered serpent god Quetzalcoatl. Once the distinctly largest known Pterosaur, since it was named in 1975, later finds in the 21st century, such as Hatzegopteryx have since rivalled Quetzalcoatlus in size.
Some estimates place Quetzalcoatlus in the 11-13 metres wingspan range, whereas others would say 10-11 metres (Hatzegopteryx is similarly debated to have either been slightly larger than Quetzalcoatlus, or essentially the same size, with estimates for Hatzegopteryx mostly being between 10-12 metres wingspan) Given that particularly large individuals always seem to emerge from out of an average size range, the difference may be moot, for the most part.
Often considered as the last wandering giants from the Pterosauria, prior to their complete extinction, the giant Azhdarchids were certainly among the very last Pterosaurs alive on Earth (albeit, that further palaeontological discoveries have revealed some other late surviving Pterosaur families, and that not all were giants - but in general, the larger Pterodactyloid Pterosaurs, including of course Pteranodon for example, were indeed alive up until the point of the K-Pg Mass Extinction event)
Generally speaking (and as alluded to strongly in Walking With Dinosaurs), the Pterosaurs had been declining up until the extinction event, and that this was what had effectively finished them off (with competition from Avian Dinosaurs i.e. the Birds, being described as the primary cause) In 1999, that was very much part of an overall understanding about the decline of numerous families prior to the K-Pg Mass Extinction event - with examples including Ammonites, Belemnites, Pterosaurs, Pliosaurs and some families of Dinosaurs.
The idea was that a combination of rising sea levels and increasing volcanic activity around the globe (going hand in hand, as per orogeny/mountain building in the ocean at the plate boundaries; the more solid mass in the ocean, literally the more displacement, raising sea levels - and the oceans covered 81% of the world 66 Ma) had severely reduced Pterosaur species, which is broadly the case in terms of fossil evidence.
However, running contrary to this, is the idea of the fossil record being incomplete in particular times and places, due to a large range of taphonomic and geological biases that may have been misleading on face value. The absence of evidence, is not necessarily the evidence of absence, as ever. In this case, though, it is generally fair to consider Pterosaurs in general to be comparatively less successful in quite as many palaeoecological niches as they had been before. Avians had begun to supplant them ecologically in some niches, yet this did not mean that they were completely redundant in some other niches.
Quetzalcoatlus has been thoroughly debated over in terms of it's feeding behaviour and role in the time and ranges in which it inhabited. Some Palaeontologists would consider it to be a far more versatile animal in terms of it's niche in the food chain, and comparisons with other Azhdarchids such as Hatzegopteryx in particular, would suggest that Quetzalcoatlus could easily be a serious mid range predator on land (when walking around on it's forelimbs and hindlimbs, as tall as a giraffe when doing so)
The idea of them being gentle fishermen wandering far and wide, has a certain appeal, though in the absence of Tyrannosaurs of a range of ontogenetic sizes, Quetzalcoatlus would be quite the formidable animal to encounter, for animals much smaller than itself. They could quite easily prey on Dinosaur hatchlings, lizards and maybe small mammals too.
Hollow boned or not, lightweight or not, they would still be striking animals when moving around on land, with their strong beaks being potentially deadly it if was hunting on the ground. That said, this idea itself has been challenged because of the abundant Theropod Dinosaurs of various sizes, typically dominating such niches on land.
The idea is that the biggest Pterosaurs may have used their sheer size and strength to scare off smaller predators from carrion, and even to potentially prey on animals more directly. Of course, large Theropods such as Tyrannosaurus, would dominate it's ecosystem on land at least, and yet perhaps younger sub-adult and juvenile Tyrannosaurs would (along with Dromaeosaurids and others) easily dominate the niches for medium sized or smaller predators, on land.
Quetzalcoatlus would have been vulnerable on land in the world in which it lived, certainly against massive carnivores like Deinosuchus (a giant Crocodilian) or Tyrannosaurus rex. Maybe their primary feeding behaviour was ichthyophagus (fish eating and therefore fish hunting) However, they were more than capable of being a serious danger to smaller animals that they could hunt for, even while on the ground. They likely flew great distances, riding thermals and convection currents, searching for food (not just fish), able to see far and wide from high in the sky.
Quetzalcoatlus featured in only a minor role in Episode 6: Death of a Dynasty, where it made a very brief appearance and was nearly eaten by a Deinosuchus. It was the only pterosaur in the episode; which is further explained to be due to Quetzalcoatlus being the last of the Pterosaurs to evolve - wandering lonely giants.
In the book however, several Deinosuchus fed on the Quetzalcoatlus as one of them dragged it to it's death.
Quetzalcoatlus might have been one of the pterosaurs flying around with the Pteranodon in The Most Dangerous Sea of Sea Monsters: A Walking with Dinosaurs Trilogy, but this is not certain and the creature never appeared in anything else. It also appeared in March of the Dinosaurs.