Nagasp
Poison
The nagasp is a pokemon that appears only briefly and a single time in the historical record, but in one of unimpeachable providence: the Verifiable Records of the king's chroniclers.
From this, we know, at least, what the king and his advisors knew of the nagasp. It was a snake of immense size that lived in a lake. Its coloration was black and light blue "in stripes", its body ribbon-like in shape, and its fangs were small in proportion to its size, but distinct in its mouth. It was said to have possessed a deadly poison so strong that it could make people melt from the inside out by breathing on them, and it was also said that locals had known of it and been making sacrifices to it for some time before this. Either or both of these things may have been an exaggeration by the local officials to make it sound more impressive, however, as the chroniclers themselves do not confirm it with first-hand observation.
Attempts were made to contract its servitude, an archaic model of pokemon capture used at the time where powerful pokemon were bargained with as much as directly battled, but they were rebuffed. Such a creature could not, of course, be allowed to operate independently of the kingdom. It represented an enormous threat to the very concept of government in such times. When attempts to convince it to come under the king's roof failed, the army was mobilized and the nagasp was harpooned, dragged a distance away to reduce the chance of its poison tainting the water supply, and summarily burned, not even keeping the skull as a trophy.
What was this nagasp? We still don't know. It may have been the last member of an existing pokemon species, an unusual mutation, or an exceptionally rare evolution. Arbok, orthworm, milotic, gyarados, drampa, a third evolutionary branch of clamperl, even dunsparce have been put forth as suggestions for its relatives.
It is humbling to think that this pokemon is known to us only by the luck of it crossing paths with a country with both exceptionally accurate record-keeping and the diligence to preserve and recopy those records all the way into the modern day. One must wonder just how many more pokemon were recorded only for those records themselves to have gone missing over the years, or where the only depiction is scrawled on a wall indistinguishable from a flight of fancy, or which never was recorded at all. The nature of missing pokemon, sadly, is there is no way to tell just how many are missing.
(naga/gasp/asp)