Man-Thing was one of the many new additions to the Marvel Cinematic Universe during its fourth phase. He appeared during Werewolf by Night and he looked amazing. It was a moment fans had been waiting for, but it wasn't the first time he has appeared in live-action.
The character actually headlined his own movie back in 2005. Yes, I was shocked when I first found out, too; because it's been so lost to history you would never even think it happened. But it did, and it wasn't very good. In fact, it was originally planned for a theatrical release but after overwhelmingly negative responses from test screenings, the decision was made to air it on the Sci-Fi Channel instead (and give it a limited theatrical release).
One thing that Man-Thing had going for it was its effects, because the title creature looked incredibly convincing (save for the odd dodgy-looking CGI branch here and there), and the horror genre worked for him too. But the script wasn't very good, the violence and maturity gratuitous and the story too logic-defying to ever be taken seriously.
The potential is still clear even when you watch it, but the finished product capitalized on very little of that.