I think this was a Tales from the Darkside episode.
There's a boy and a hairy monster who are trapped in a basement by a woman upstairs. They begin to plot an escape.
I remember watching this as a little boy but never saw the ending. Is it a Tales from the Darkside episode or some other anthology series?
You're talking about "The Gift" - Here's what Wiki says about it...
Sid Dolan (Abe Vigoda) and Kirby (Brad Greenquist) have kidnapped rich kid Jeffrey and taken him to an unheated mountain hideaway in the snow. While searching the basement for warm blankets, they discover a monstrous, furry Beast (Carlos Lauchu) in the cellar. Dolan wounds it with his gun, and then the kidnappers chain boy and beast next to one another in the basement. The Beast makes telepathic contact with Jeffrey, explaining he is a good being who can shape-shift. Jeffrey and the Beast learn that they are to be killed. The Beast offers Jeffrey a way out, but the choice is gruesome.
Care killed the cat, but satisfaction brought it back
It's actually an episode of Monsters, not Tales from the Darkside... The Gift
But they're not trapped by a woman upstairs, so it may be something else entirely that the OP is looking for.