In Monstrilio, the line between human and monster is often difficult to define. The novel undermines the clear distinction between humanity and monstrosity by showing how monsters can be human and humans can be monstrous, thereby challenging the idea that humanity is by definition separate from monstrosity.

Monstrilio is the novel’s central “monster,” yet this classification stands in stark contrast to his defining characteristics of innocence, gentleness, and general bewilderment regarding his human body and the expectations that accompany it. As the lung starts to grow into Monstrilio, the creature is violent and carnivorous, but there is also an element of innocence that softens his monstrosity. When the lung attacks Lucía’s dog, he “acted without malice” (57); he was simply hungry and did what was natural to him. As Monstrilio gets bigger, he sometimes wreaks havoc, slaughtering neighborhood pets or slicing open Joseph’s arm with his sharp claws. However, Magos and Joseph don’t fault him; they remain convinced that Monstrilio only acts this way because “[h]e doesn’t know any better” (141).

Magos, Joseph, and Lena all love Monstrilio, yet Lena disagrees with Magos’s insistence that the creature can “learn” to control his impulses. Lena loves Monstrilio precisely because he is a monster, challenging the assumption that humanity and civility are inherently superior traits. The central question of the novel then centers around what Monstrilio is “supposed to be” (148). Anxious to make Monstrilio less “wild,” Magos wants to cut off his arm-tail. However, Joseph and Lena worry that his wildness is an important part of who Monstrilio is: Without his “Monstrilioness,” Joseph fears he will be simply “an empty husk” (197).

With his arm-tail amputated, Monstrilio does, in fact, grow into a boy-shaped body, but Magos’s assumption that this would be an improvement is misguided. Instead, Monstrilio’s new shape comes with a whole host of human expectations that continually confuse and frustrate him. Like Monstrilio, the boy-shaped M is “hungry all the time,” but now he “know[s] he shouldn’t be” (273). Becoming more human does not eradicate M’s monstrous impulses, but makes them seem more wrong. Nevertheless, M tries to be the boy his parents want him to be, exhibiting a sense of empathy and emotional intelligence that contrasts with his monstrous tendencies, pretending, for example, that he isn’t hungry because he can see Magos and Joseph are pleased to think he is “getting better” and “[m]ore capable of being human” (288).

In the end, however, M recognizes that being human is a charade he cannot sustain. By doing what they believed was best for M, Magos and Joseph forced him to suppress his true nature, yet M’s physical body begins to rebel. His arm-tail begins to regrow, along with the fur that covered him as Monstrilio. Stepping into the darkness, M is able to release the human expectations that are not his “to carry anymore” (324), finding freedom and release in embracing his true monstrous self.