The Avunculus horrendous (pl. Avunculi horrendous) was a large, round, porous creature that dwelt in swampy areas of the planet Redeklis. The creature was roughly spherical and emitted smoke from its upper hemisphere and gas from its lower. It was mostly sedentary, though it could move short distances using small bumps that protruded from its lower surface. A larger bump on its top surface was reminiscent of a head and did, in fact, contain most of the creature's sensory organs.
The Avunculus horrendous acquired its food by targeting a passing creature and wafting smoke and gas in its direction. The noxious substances rendered the prey immediately unconscious, and it remained so long enough for the A. horrendous to waddle over and consume it.
When I first saw pictures of the Avunculus horrendous, I was immediately reminded of my terrible old Uncle Vic, who would arrive unannounced, stay for weeks on end, fill the house with noxious cigar smoke, and constantly call out to me, "Hey, kiddo. Pull my finger." After his departure, it often took weeks for the house to smell normal again.
It remains unclear what purpose the Avunculus horrendous served in its environment. It consumed other creatures, emitted smoke and gas, and did precious little else. Much like Uncle Vic.
Flatulence, it seems, is not an uncommon theme in species extinction, and so it was with the Avunculus horrendous. My research reveals that technological advances by the Redeklans caused an increase in the amount of pollution in the atmosphere, which reduced the amount of sunlight available to plants. This caused some of the hardier plants to become even tougher and more fibrous. This in turn increased the fibre in the diets of the creatures that ate those plants. That high-fibre diet was then passed on to the Avulunculi horrendous, who would eat just about any creature at all.
It was only a matter of time.
Then, the inevitable happened: Someone lit a match.
Don't misunderstand. Matches were not uncommon on Redeklis. It's just that explorers venturing into the swampy areas knew better than to use any kind of open flame for any purpose.
Except for this one guy.
The resulting fireball swept across the entire surface of the planet, burning everything, animal, vegetable, and mineral alike, to a crisp.
And that, as Uncle Vic was wont say after watching football on the telly, was one helluva match.
Status: Burned to a crisp
Right Thing to Say Should You Ever Meet One: Oh, sorry. The guest room's been converted to a library.