Eh, IY is half human and more attached to his human half than his youkai half.
Sesshoumaru, OTOH, would not be too out of place in the Faerie Courts of the Dresdenverse, possibly as an unaligned Lord/King of Summer with a focus on Dogs and poison, especiallyafter coming to terms with Tenseiga and purifying the souls of the damned. Not actively malicious, but inhuman, very dangerous to annoy and incredibly powerful.
I dunno who is behind the modern concept of faerie as cute little Tinkerbell pixes, but when you read a story that deals with them in their classic roles as inhuman, often amoral beings with odd/immense power and a tendency to wreak generally randomized havoc and often lethal mischief while going about their day to day affairs, it's hard to say that the Fae are anything less than completely terrifying. If Peter Pan had a faerie from the Dresdenverse's Unseelie Court or a Discworld elf following him around, suicide might be a good idea (though there are some that aren't so bad, and the original Peter Pan was kind of psycho himself and the Fox Peter Pan was basically a very low level Fae Lord to Neverland).
Demons are scary and all, but they tend to seem too organized and specialized towards stuff about Hell and souls to accurately encompass youkai. If anything, Demons could arguably be considered a subset of youkai, like the various youkai of Hell from Inu Yasha.