What's The Deal? That Time When Spider-Man Turned into Man-Spider?

The Marvel Classics Man-Spider Toy Biz 6" Action Figure

I recently sold mine to someone online and thought I would learn more about it before I gave it away.

Man-Spider is the mutated form of Spider-Man which appears in both the comics and in the 1990s animated series.

I got the Marvel Classics Man-Spider Toy Biz Action Figure from K-Mart back in 2000. I thought it looked cool I guess.

I recently sold it to someone online and thought I would learn more about it before I gave it away.

So here's what I found out.

It looks like the "The Six Arms Saga" is where this began I think? It is a story arc written by Stan Lee and drawn by Gil Kane. It spans the issues Amazing Spider-Man #100–102 (1971) and features the first appearance of Morbius, the Living Vampire.

In the story, Spider-Man is swinging through the city with four extra arms.

Basically, Spidey brews up a chemical cocktail intended to terminate his spider powers. Peter takes the cocktail and falls into a troubled sleep in which he fights all his enemies while enduring excruciating side pains. When he wakes up, he notices a ghastly thing: he has six arms. The potion has increased his spider powers rather than nullifying them.
From the Villains Fandom Wiki - Man-Spider:

In Spider-Man: The Animated Series from the 1990s
Spider-Man's transformation into the Man-Spider was not caused deliberately, but as a result of his body mutating further from the original spider-bite that gave him his powers.

His initial attempt to cure himself resulted in him growing the four new arms, the accelerated mutation subsequently causing him to mutate into the Man-Spider.

Spider-Man was returned to his human form but had to receive special treatments from Doctor Curt Connors every 24 hours to prevent the mutation from reoccurring.

In the cartoon - Man-Spider was shown spitting acid from its mouth and able to produce webbing from every limb without web-shooters.

The comic book version - He never displayed any noticeably enhanced strength nor acid-spitting; however, it retained Peter's consciousness.

More on Wikipedia:

In the Mutant X universe, Spider-Man still has six arms. For unexplained reasons, he has taken to calling himself Man-Spider instead of Spider-Man. Moreover, the chemical he consumed apparently altered his DNA.

Here is a YouTube video about Man-Spider