Actually, the German translation for "wyvern" would be "Lindwurm", and in common mythology those two creatures are virtually the same, except for the wings.
Artistic license sounds like a very likely explanation for the Lindwurm's wings as depicted on the Klagenfurt crest - just like the number of the Lindwurm's legs changing from two to four some centuries ago.
As to the Tatzelwurm stories: The first scientific work mentioning this lizard-like creature was the
Historica animalium by the Swiss naturalist Conrad Gessner, written in the 1550's. Even though Gessner's writings became standard reference works throughout Europe in the 16th and 17th centuries, and are now seen as the starting-point of modern zoology, the
Historica animalium also featured animals like the unicorn and the griffin and therefore can't be seen as a proof for the existence of the Tatzelwurm.
But especially between the 19th and the early 20th century there have been dozens of people who claimed they'd seen something like a Tatzelwurm.
The creature is usually described as being between 30 and 90 cm long, with a somewhat cylindrical body shape, and a short and thick tail.
The creature's "cat-like" head is heavy and blunt-nosed, and there's no narrowing at the neck. Note that the word "cat-like", as it is used by some of the witnesses only refers to the rounded shape of the Tatzelwurm's head - as opposed to the long nose a lizard's or snake's head usually has.
The Tatzelwurm has no ears, round "evil" eyes, and seems to have scales, the colour ranging from a greyish white to greenish to black. The creature has only got two small, short front legs (even though some say they saw two even smaller hind legs).
A lot of people said that they heard the Tatzelwurm hiss like a snake, and that it was so aggressive it attacked several times before disappearing somewhere.
(In Tatzelwurm legends it is said that the Tatzelwurm can jump a few meters, and that it's so venomous "its breath alone can kill you" - but these "facts" sounds like exaggerated superstitions and have never been reported by any of the witnesses).
There are theories that the Tatzelwurm is some kind of burrowing lizard or maybe even an amphibian, because it is said that the Tatzelwurm is most likely to be seen in the first rain storm after a long drought.
Needless to say that nobody ever captured a live specimen of the Tatzelwurm, and no skeleton has ever been found. The last sightings were reported around 1930, which has lead to the belief that even if there really was such a big reptile-like creatures in the Alpine regions, it may be extinct now.
There have been lots of theories trying to explain the Tatzelwurm phenomenon, for example that maybe the Tatzelwurm sightings really were about stray otters or mangy badgers and not lizards.
Other, more cryptozoological theories about the Tatzelwurm link it with animals such as the
Gila monster (heloderma suspectum), some
burrowing worm lizards, the
skink (tiliqua rugosa, for example) or two-legged amphibians such as the
Greater Siren (siren lacertina).