The transition scene is everything. A good story spends chapters on the subtle creep of equine instincts, not just the hooves and tail. It's the protagonist fighting the urge to graze on a neighbor's lawn, or feeling a surge of pure, inexplicable panic at the sight of plastic flapping on a fence. That internal monologue fracturing, becoming simpler, more sensory. I read one once where the guy kept trying to hold a pen and his fingers just wouldn't work right anymore, and he started crying from frustration. That hit harder than any overtly sexualized transformation. The struggle isn't always violent; sometimes it's this heartbreaking resignation as human memories start to feel like a story someone else told you.
A lot of newer stuff skimps on this for quicker... payoff, I guess. But the older forum serials, the really gritty ones, they'd make you sit in that itchy, confusing middle ground for ages. You'd feel the character's dread and the weird, unwelcome thrill of the new instincts together. The best handle it by making the animal side not evil or base, just different. A different operating system booting up and overwriting the old one, with constant file corruption errors in between.