Can't understand why people love to fuck cartoon horses.
Pretty sure it's the same way people become gay/furry/train'y/etc. Some unfortunate people encounter real humans who reject them/ hurt them/ create other negative associations with that specific group of people (blondes/ opposite sex/ humans in general), thus these unfortunate people become less attracted to, or even afraid of shitty human groups; while simultaneously they receive positive associations with humans of same sex/ cartoon characters/ pleasant means of transportation/ etc, thus becoming more and more attracted to same sex humans/ horses/ trains/ etc; this attraction eventually transforming into a sexual one. As a human, I always consider myself to be one, and am forced to search for another human to mate with. However, as a human, I am also forced to believe that humans are the superior species, the "ultimate good". At times when every human female is shouting at me or telling me to go fuck myself, it is truly difficult to think of them as "good", and, therefore, "human", an entity of the same species as I. At the same time, I watch cartoon horses, who interact with one another with love and respect, acting similarly to me and expressing similar emotions as I am feeling. That's how horses may, in one's cognition, become more human than real humans, and therefore, more sexually attractive.
Furthermore, sometimes people develop fetishes which are impossible to enact in real life as a defensive mechanism of sorts. The need to be loved is impossible to get rid of, but for some people it is also unachievable; so they get stuck in cycles of desperation and rejection. Our psyches can decide to stop this pain by losing sexual attraction to that what should be (but isn't) possible to fuck IRL and replacing all related fantasies and planning with those that are not only unachievable, but impossible to try (fucking a sentient train), thus keeping us in constant "planning" phase, without ever getting to rejection part.
Personally, I almost became a full brony at one point in my life, but occasional non-disgusted "hello"s and small talk with women saved my soul. I do consider horses to be better humans than humans and do find their porn (done right) aesthetic, but have never used any as fap material, TBH.