Regeneradores were experimental bioweapons created by the Plaga implantation procedure. Capable of near-instantaneous regeneration, they were able to take incredible amounts of damage before succumbing to their injuries.
The first one is encountered in Chapter 5-1. When Leon S. Kennedy collects a keycard from a dead body, a Regenerador awakens in a nearby locked autopsy room. The Regenerador breaks down the door and attacks Leon. First time seeing it, most players run away because they are terrified of the creature's appearance and its (seeming) invulnerability.
Two more are met outside of the Autopsy Room; one around the second corner of the corridor, and another through the iron gates down a very tight corridor at the very end to the right past the second. These ones are encountered after the Autopsy Room. If the player kills the first one in the corridor, it makes the second easier to deal with. Don't try to deal with both of them at once; it is very difficult and will likely result in the player's death.
The fourth is located in the room where the freezer key is converted into the waste disposal key. This one only attacks if Leon unfreezes the room using the control panel and picks up the Infrared Scope or enters the room with an Infrared Scope carried over from a previous game. With this Regenerador, the door locks, forcing the player to battle with it until it is dead, or until three minutes are up. The best way to battle this Regenerador is with the infra-red scope the player just picked up. The door unlocks and the player leaves with their best defense and offense against three other Regeneradores.
The next two Regeneradores the player find will be when the player is with Ashley just before the wrecking ball part. The first one will come out when the player calls to Ashley or pulls the first lever. As it is a normal Regenerador, it has the plagas you must shoot before it dies. The second one is further down the corridor, which will come out when Leon calls to Ashley or walks close enough until it is visible. It is not advisable to run past the first Regenerador and into the second one, because you will end up having to fight both at once.
Prison Cell (Island)
The only encounter Ada Wong has with a Regenerador is right before her fight with Jack Krauser, where she sees one lying on a table; this one is harmless. This is also the last Regenerador Leon has to fight. The last Regenerador is in a prison cell; right when the player walks in, they can hear unsettling music and erratic breathing. The Regenerador is in the last left cell door. Most players will walk right by if they don't hear the breathing and are startled when a Regenerador is biting their neck a second later. Another way the player can get to this Regenerador is by hugging the left side of the prison door; the player can see that the Regenerador is just standing there. It will only start to notice the player if they run out, shoot it, or make it regrow a limb. Also, the player can run by the door and climb up the wall - the Regenerador will follow slowly, running into the wall you jumped up. The player can kill the Regenerador that way or just leave through the door up there.
Regeneradores have a different number of Plaga in them with each difficulty setting, ranging from 2-5. The second species of Regenerador, called the Iron Maiden, always have one on their back. The Player will have to either maneuver around one of them quick enough or shoot one leg off so the plaga on the back is exposed to the Infrared Scope after it falls to the floor.
↑The Regenerador is mistakenly referred to as the "Regenerator" in the English translation to Resident Evil 4, a simple mistake given the very slight difference between the Spanish word and its English equivalent. The "-dor" spelling is used everywhere else.
↑Transcript from biohazard4 kaitaishinsho, page 124: "ひとりの人間に対し複数のプラーガを寄生させる実験によって生み出された怪物｡実験の結果､たとえ身体の一部が欠損しても瞬時に再生できるほどの異常な代謝能力を獲得した｡しかし､ 人の形をとどめてはいるものの､ 肉体構造は完全な別物に変してしまっている｡こうした変質は脳にまでおよんでおり､ 人間としての思考能力はまったく残されていない｡"