People in general think we just make it up for attention. That the most visible part of our community are teenagers makes it seem like a childish phase. They also believe that if it's real, it should be considered a mental illness. So, look at it from their perspective. "There's teens online who pretend to mentally ill for attention, and they compare themselves to trans people or claim that it's a spiritual belief."

Of course there will be some people who, based on that misunderstanding, will dislike it.

There's also the general dislike for anything weird (people have bullied emos, furries, pagans etc for similar reasons), and the idea that humans are separate from, and better than, other animals. In some cultures, it's considered insulting to compare humans to animals (and this is why evolution can be hard for them to understand), so a person willingly calling themselves a non-human animal seems ridiculous and disgusting. Keeping up boundaries between categories, such as man/woman, human/animal, adult/child is a recurring theme in many rituals, traditions and laws. People who blend those categories tend to be looked upon with disgust, which is why transphobia is thing, for instance. Liminal categories are, by definition, controversial.