The Skrath is indeed horrible, and feared in every corner of every land, more even than the dread Tarrasque. His body is the loathsome body of a hideous crawling thing, and his face is a grotesque mockery of the faces of men.
He creeps along the undersides of carriages, whispering foully to coach-horses and exposes his horrific tongue to lone fishwives on the wharves in the night.
It is said that the coming of The Skrath makes doves abandon their eggs, it is said that the coming of The Skrath causes gourds to grow in unclean shapes, it is said that the coming of The Skrath makes good men weep and wail and gnash their teeth and clean their knives until his coming is ended, it is said that the coming of The Skrath bodes ill for the education of children, yet it is also said that he teaches them the secrets of the Moon and the White Star.
______ Crunch & Notes:
Basically, The Skrath is a unique three-foot-long greyish-green monster that shows up one day in a town or city and is usually found just lying there doing nothing (there is a 25% chance it will be found lying there doing nothing with its tongue hanging out). Treat The Skrath like an artifact or relic in AD&D--what exactly The Skrath actually can do is left entirely up to the DM, but the main thing is that the villagers, townsfolk, nobility, and other excitable 0-level types see The Skrath as the worst possible omen and have copious legends built up around it that may or may not be true and are terrified of it.
In imagining possible properties of The Skrath, remember that The Skrath may be powerful but it is not flashy: it won't just shoot fireballs at people, but maybe anyone who points at it will slowly melt over the course of a day, or maybe The Skrath just makes everyone roll at -15 all the time, or maybe it eats kindness and so removes all tender feelings from people, or maybe it is impossible to kill not because it has a million hit points but merely because anyone wanting to kill it immediately loses all motivation, or maybe it makes everyone's wishes at the exact moment it appears come true--so if someone's thinking "I wish I could find my shoe" then s/he'll see The Skrath and find a shoe, and if someone's thinking "I wish I'd never had a child" then their child disappears, or maybe The Skrath is a benevolent wizard doomed to crawl the earth in a detestable guise, or maybe The Skrath is a magical rorschach test that responds to people in whatever way they treat it, or maybe The Skrath is a philosopher that says disturbing things like the Earth revolves around the Sun and monarchy is not the ideal form of government and women should have the same legal rights as men, or maybe The Skrath is just a one hit-die greyish-green animal that lies there looking horrible and not doing anything and people think it's evil even though it's actually just ugly.