There is no "alt-left". It's a term made up by the right to far-right in an attempt to create a false equivalence between white supremacists and those opposing white supremacism:
It's a "made-up term" used by people on the right to "suggest there is a similar movement on the left," [Oren Segal, director of the Anti-Defamation League's Center on Extremism,] said.
But there's no equivalent with the anti-Semitic and bigoted groups that call themselves "alt-right", he said.
[George Hawley: ] "There is no such movement as the alt-left. Obviously, there are left-wing extremists but there is no congruence between the far-left and the alt-right."CNN: What's the 'alt-left'? Experts say it's a 'made-up term'
Researchers who study extremist groups in the United States say there is no such thing as the “alt-left.” Mark Pitcavage, an analyst at the Anti-Defamation League, said the word had been made up to create a false equivalence between the far right and “anything vaguely left-seeming that they didn’t like.”New York Times: Alt-Right, Alt-Left, Antifa: A Glossary of Extremist Language
Ultimately, the intent seems to be to frame alt-left as the opposite of alt-right and create a false equivalence between groups on the far ends of the right and left. But here's the thing: No left-wing group has ever called itself the alt-left. And the groups smeared by the alt-left label don't include anything like the heinousness of overt white supremacism that has increasingly defined the alt-right.
It's a blanket term some right-wing media commentators and white nationalists have taken to throwing over groups they disagree with Wired: There Is No 'Alt-Left,' No Matter What Trump Says
"Alt-right" on the other hand is a self-applied term that was made up by white supremacists themselves for propaganda purposes.
There are of course far-left ideologies such as various sorts of communism, anarchism, etc. They are not comparable to the so called "alt-right" though; while some may contain racism or antisemitism, it is not central to any left-wing ideology, while they are central elements of the so called "alt-right". There are of course a lot of other differences, but that seems to be outside of the scope of this question.
