A couple more, not all from my horse:
Killer Butterflies: Butterfly landing on nose led to violent head-shaking and a near-bolt, and a sideways scoot every time the horse passed that place in the arena.
Unattended Running Tractors: If it’s not attended it could shift into gear and chase horses
Newly painted divider line on the road: Obviously a very long yellow snake (and G*d help us all if it had been a double yellow line!)
White lines left by retreating waves: OK, the beach is full of scary stuff, but how about a very brave eventer who spooked at those little white lines (snakes again), then JUMPED over them and was perfectly happy to go into the ocean up to his belly?
Bicycles: Being ridden, even coming up from behind, no problem. Being walked, or left unattended, um, very scary!
And one that I consider to be completely reasonable:
Three Jack Russell Terrors barking and growling as they come bounding across a paddock toward us.