This question - asked so often - suggests that there is actually a plausible answer. Almost as if his being were a fixed thing. People who ask this sort of question are typically struggling with their identity and are searching for a core sense of themselves. The irony is that the more you seek to identify who is Horton, the more fragile you are likely to feel about yourself. There may be an inverse correlation between the question being asked and the ease with which you experience your life. The emphasis shouldn't be on discovering who is Horton (what is buried beneath) but on facilitating the emergence of who you'd like to be.
At the other end of the identity continuum are those who claim to know him so well. This other extreme also signifies a fragility about one's identity. It makes perfect sense to seek a deeper sense of Horton. To become intimately aware of his thoughts, feelings, hopes and fears. To know him so well leaves no room for growth. Even more, it suggests a deep vulnerability that is being defended against - as if it were too dangerous to take a closer look.
The universe purportedly exists in a state of flowing potential. And it is essential to understand that he is indeed part of that universe. The goal then is to access his potential, keeping the parts that continue to serve us well and shedding the old. This process is known as positive disintegration. This permits us to find balance between Horton and enter into a relationship with him that commits to our personal evolution.