Sauron's Ring wasn't the first piece of jewelry to start a massive war in The Lord of the Rings, but it was the most powerful piece of magical paraphernalia in J.R.R. Tolkien's universe. Sauron used it to regain physical form after his body was killed during the Fall of Númenor, and then, he used it to conquer almost all of Middle-earth in the Second Age. When Sauron lost it during the Battle of Dagorlad, his body was killed again, and his spirit was forced to flee the battlefield.
After Sauron lost his Ring, it came into contact with a number of powerful figures. Gandalf, Elrond and Galadriel could have claimed the One Ring as their own, and each of those hypothetical situations would have ended badly. Their natural power -- combined with the power of the One Ring -- may not have been enough to conquer Sauron, but it would have doomed Middle-earth. But what would happen if Treebeard had happened upon the One Ring? Would it have corrupted him too, or would he have been oblivious to its effects like the powerful Tom Bombadil?
What Were Ents in The Lord of the Rings?
Of all the creatures in Middle-earth, the Ents were some of the most tragic. They were created after the Valar Yavanna requested that something be made to protect her trees from Aulë's Dwarves. So, Eru sanctioned the creation of the Ents, and their sole purpose was to herd and protect the trees of Middle-earth. They were completely sentient, but they looked like the trees they protected. And like trees, they grew slowly over the ages. In fact, Treebeard was said to have been the oldest being in Middle-earth besides Tom Bombadil.
Merry and Pippin encountered Treebeard in The Two Towers. After a lengthy conversation with Treebeard and initiating an Entmoot, the two Hobbits convinced the Ents to attack Isengard. While Saruman watched from Orthanc, the Ents destroyed the Orcs' equipment and broke the dam above the tower, which sent the River Isen flooding into the valley. The attack showed the physical strength that the Ents could command when sufficiently roused.
Treebeard Taking the Ring Would Doom Fangorn
Inspired by this Quora thread, it's interesting to think what Treebeard would have done with Sauron's One Ring. Maybe the Ring didn't fall into the Anduin River; maybe it fell into the Entwash, where Treebeard could have found it. In that scenario, Smeagol would never have turned into Gollum, and Bilbo would never have taken it from him. Instead, Treebeard would have claimed it for his own. Because of his long life, the Ring wouldn't have had any immediate effects on Treebeard, but the Ring would start working its evil purposes.
Over time, the Ring would corrupt Treebeard, and eventually, the effects would have been visible in his forest. Just like how Greenwood the Great transformed into Mirkwood, Fangorn would have turned into something dark and evil -- which would have attracted the attention of Sauron. Seeing the effects of the One Ring, the Dark Lord would have sent a massive force of Orcs to raze Fangorn. With an immediate threat to his forest, Treebeard would have been forced to fight.
Treebeard would have raised the Huorns, and there would have been a terrible battle. However, Treebeard would be severely outnumbered, and despite his physical strength, he wouldn't have known how to use the Ring in combat. So, Treebeard would have failed, and Sauron would have reclaimed his precious Ring. Thus, Treebeard finding the One Ring wouldn't have ended well because the Ring would have corrupted Fangorn forest just to attract its master.