Fetishes are by nature a product of the sexualization of a non sexual thing for sexual gratification. An easy one to explain is urophila or the fetishistic sexual obsession with urine, there are many versions of this paraphilia but chiefly it's thought it is caused in some people who are raised in extremely religious households where the rare time they can touch their genitals or explore their sensations is during urination therefore at the onset of their sexual awakening they associate urinating with the ability to even momentarily touch their genitals and possibly engage in a quick act of masturbation these episodes continue and out it grows to where the individual associates urine with sex and will seek out a partner to engage with them and play out that fantasy. Feet maybe in a society like most Western cultures where we usually wear a foot covering like a shoe rhat covers the whole foot and that seeing people nearly naked in swimsuits at a beach including their feet may initiate a sexual feeling in an individual that eventually becomes a foot fetish, of course there are other possibilities too. Recording them is as old as the written word, several ancient texts talk of people and their sexual proclivities including punishments for some, even the Bible has rules against things that are fetishes and notes them. Kraft- Ebing like you say was one of the first if not the first to write it down and study it, although his ideas were very much like many others of his day, incorrect and subsequently disproven. He thought in the same vein as the Church that sex was an act for procreation alone and that any other sex was a perversion and may require "treatment".