I first played Altered Beast in 2009 on a potable Mega Drive console which came with this game as well as a few others. I also have the ROM for the game on the Wii. Altered Beast is a beat-em-up game which is centered around a centurion who has been brought back to life by Zeus to save his daughter Athena from the clutches of Neff. Each level has you going through an auto-scrolling level where you beat up enemies for points. You can also obtain power-ups from defeating blue pigs which occasionally run past you. These power-ups make you much stronger until you gain a transformation which can turn you into mythical creatures like a dragon or a werewolf. Keep in mind that you are not invincible during these transformations and you only lose this transformation once you get a Game Over, so do not worry about losing it too excessive damage or a loss of a life.
This is a very short game, with only 5 levels in there before reaching the final boss of the game. There are a number of mini-bosses after each level and you need to be in a transformed state to activate these bosses. The Genesis/Mega Drive port of this game did not get the best of reviews, as there were some audio issues with the game during translation. That and the game did not age very well as a beat-em-up. But those flaws will not stop me from playing this game. If you are looking for an older beat-em-up game to play then you should give this game a try.
Here is the YT link to the famous line said in the beginning of the 1st level: Altered Beast Clip