The Utility Monster is an often used argument against Utilitarianism, based on the notion that if a being existed that gained more utility per unit of pleasure than other beings, Utilitarianism would logically entail that we should sacrifice everyone else to feed this monster pleasure. Most Utilitarians dismiss this argument on the grounds that such beings do not and cannot exist.
While the specific Utility Monster of Nozick, which didn’t suffer from diminishing marginal utility most likely does not exist, the more general kind of Utility Monster who merely gains more pleasure than everyone else, does in fact exist, and we are it.
See, human beings, as far as sentient beings are concerned, are capable of experiencing pleasure at a higher level than other animals. Our highly developed brains also us to cognitively experience far more, far more deeply than other animals. A pig can never enjoy a good novel.
In fact, it’s arguable that more intelligent species are utility monsters to less intelligent ones. Thus, the utility monster argument actually ends up justifying a kind of speciesism, in the form of a hierarchy of sentience and intelligence.
Furthermore, and possibly more ominously, it argues that if the Technological Singularity were to occur and produce a God-like super A.I., that A.I. might very well be a Utility Monster to the human race.
Note however that the law of diminishing marginal utility tends in practice to mitigate the dangers of Utility Monsters. Even if initially a Utility Monster might benefit more from utility than other beings, diminishing marginal utility would eventually cause the benefit to fall to the point where other beings would still benefit more than the Utility Monster after a certain amount of utility was spent on it.