Man-Thing first appeared in May 1971 in Savage Tales #1, created by Roy Thomas, Gerry Conway and Gray Morrow (and Stan Lee). Swamp Thing debuted in House of Secrets #92, created by Len Wein and Bernie Wrightson, in July 1971.
But before either of them, in The Incredible Hulk #121, Roy Thomas and Herb Trimpe created “The Glob.”
The issue hit the stands on August 12, 1969–nearly two years before Marvel and DC introduced the more famous swamp characters (within mere months of each other).
The character was a thief who escaped from prison and got caught in a radioactive swamp. He had almost no brains left, but was sympathetic because his wife was dying. The look, the origin, the character, the power-set, all extremely similar to the Things.