Playing on the love of horses:
* Neigh-sayer: A play on the horse's sound and the idea of being overly enthusiastic.
* Hay-Head: A bit silly, but implying they are constantly thinking about horses.
* Maneiac: A bit dramatic, but funny if they are overly passionate.
* Stablemate: Implies they spend more time with horses than people.
* Horse Whisperer (in a sarcastic tone): Plays on the idea of being overly sensitive to horses.
Playing on the lifestyle:
* Giddy-up: A bit juvenile, but amusing if they are always rushing to the stable.
* Bridle-path: Implies their life revolves around horse riding.
* Hay Bale: Implies they're always covered in hay, like a real horse person.
* Barn-burner: A bit dramatic, but funny if they're constantly on the go.
* Hooves-up: A pun on "hands up" that suggests they're always ready to ride.
Remember: These are meant to be lighthearted and fun, but it's important to consider the person's sensitivity and whether they'd find it funny or offensive.