The first thing you need is for a given horse to know on what slope it is standing. All depends on how you display/create your slopes though, but we can assume the horse is overlapping with the floor. If so, an event "Horse is overlapping with Slope" could let you adjust the angle every tick.
Now that you know what slope the horse is currently on, you need to add some sort of variable on every slope that stores the angle at witch the horse should be when it is there. Let's say you have this variable "horseAngle" on the Slope object.
Now your event is :
Horse is overlapping with Slope => Horse set Angle to Slope.horseAngle
Of course, you will need to prepare this variable on each and every slopes.