Basically where monsters in RPGs can fight alongside the heroes. Allied monsters' individual personalities may exist (Paper Mario) or not (Dragon Quest series to an extent). It's more likely in the latter case that defeating them first is a requirement for them allying with you--though, it's not unheard of in the former case. There is usually a Slime/Goomba type monster ally that can/will join earlier than other monster allies; they're quite likely to be a Lethal Joke Character. Examples of Monster Allies include: