Wyverns are an exception to the general rule that SLASH'EM's new monsters tend to be more dangerous than their vanilla counterparts. As compared to normal dragons, this is a relatively weak monster. The poison component of its attack is rendered mostly harmless by MC 3, and completely so by poison resistance. By the time one encounters wyverns, one should almost certainly have at least one, if not both, of these properties. Since they have no breath attack, they are thus less of a threat than normal dragons.
A wyvern is a dragon-like creature from English mythology that possesses only two legs (sometimes none) and a barbed tail, unlike most depictions of dragons.