Easiest thing is to slow down, and call out something like "Hello, cyclist behind, ok to come by?" Mostly, you're alerting the rider to your presence (if you're oncoming towards them, the rider at least sees you sooner), and the horse to the fact that you're human. Remember that horses are prey animals, and programmed to run/kick first and ask questions later. Coming up quietly, and then suddenly appearing in their field of view is exactly what a predator would do. They like to be able to see you head on and work out what you are.
On a recumbent, it's even worse, because most horses, like most humans, very rarely see them (and I reckon the low long shape is even more 'predator' shaped). I tend to slow down, atttract the rider's attention and then talk comically to the horse - stuff like "it's alright horse, I'm human, I know I'm a funny looking one... etc" Generally gets the rider laughing. If it comes to it, I'll stop and let them bring the horse past. It's all just give and take - sometimes they will back up or pull into a field to let me by.
I'd say that 90% of the horses I encounter being ridden are fine, just the odd one refuses to come or hesitates. Horses tethered by the roadside tend to be able to stand and watch you coming and work out that you're not a threat - a call out gets their attention too.
Funnily enough, in France last summer I noticed that horse in fields tended to gallop towards the hedge or fence to get a good look at us (8 recumbent trikes), rather than away.
