Magic's Pawn by Mercedes Lackey

He's Vanyel, the whiniest teenager ever chosen for a top government position by a horse! He's Tylendel, doomed twin and bad decision maker! Forget it, Jake, it's Valdemar! This is the first in Mercedes Lackey's Last Herald Mage trilogy, Magic's Pawn - it is not a romance novel, there is no happy ending, there are only tears. The tears are, in fact, the point. It's the first book a young Sara ever read about love between two impossibly beautiful young men, plus it has omg horsies - so yes, I imprinted on it like a duckling. There are some pretty intense topics in this book - it's got really heavy suicidal ideation and suicide, it's got a bitchy horse-what-ain't-no-horse who also dies by suicide, it's got some bad family business and some pretty intense homophobia, it's got some Mystic Native tropes, and if you read the other books in the trilogy you're going to run into some truly traumatic sexual assault and incest. Whee! As promised, here's a write-up of Lackey's wild ride at Dragon Con in 1997! It really does involve a man who calls himself Pony White claiming he got attacked by ninjas. You gotta read the whole thing to get the whole story. From the same source, if you're dying to know what "filk" is, here's a pretty good explainer with examples.