When Muammar Gaddafi was cornered in a sewer pipe by Libyan rebels, when they got him out they proceeded to beat the living daylights out of him.
Many gangs use these as a method of testing their new recruits (also considered an initiation ritual). If they like the new guy (or girl), they tend to count at a normal rate so their candidate doesn't get too beat up. Others might get a slower count, if they're allowed in at all. The challenge is staying conscious and alive — it isn't uncommon for prospective gang initiates who are "beat in" to die this way due to internal injuries sustained by the beating.
Fights in the NHL generally avert this - it's usually difficult to land serious injury with your punches when both you and your opponent are on ice skates and have to grab each other's jerseys just to stay close enough to have a fight. Referees are generally told to let it play out if it's too dangerous for them to intervene. However, if one player falls on his back onto the ice it can quickly become this trope because now he has no escape and is in a vulnerable position - referees will intervene quickly, especially if there's high emotion involved (i.e., a dangerous boarding check).
Chimpanzees, when the males are on patrol, are masters of this. If they find a chimp from a rival troop in their territory, they will rush it, gang up on it, and inflict terrible brutal damage on it, including but not limited to dragging it around by its genitals (if male), yanking the limbs and throwing the victim around by them, severe beating with muscles four times stronger than a human male'snote if said human isn't in fight-or-flight mode, that is, and using those large strong canines to inflict horrendous wounds. Even a lone chimp is capable of doing this, especially to humans; victims of chimp attacks are sometimes mauled so badly, their entire face requires extensive reconstruction (as an enraged chimp alwaysgoes for the eyes), and sometimes even that can't be done.
The Roman discipline practice of decimation, or "removal of the tenth" amongst the Legions. A Legion charged with a serious offense, such as losing its Eagle standard of fleeing the battlefield, would be divided into its constituent ten man groups (known as a contubernium) and then a lottery was held. The soldier selected by the lottery would be executed by the other nine soldiers of his unit. Often, this would be carried out by stoning or clubbing. Decimation was practiced only for the most serious offenses, thus it didn't happen often. It should be pointed out that the contubernium wasn't just an administrative division, it was a practical unit, each contubernium was a squad of men who shared the same tent. Legionaries suffering decimation wouldn't be killed by just anyone, they'd be killed by their friends.
In 1856, Representative Preston Brooks infamously beat Senator Charles Sumner senseless with a cane for criticizing a family member over a feud on slavery. It was so brutal Brooks accidentally hit himself with one of his backswings.