A few thoughts to consider;
1) Start way before sunset - animals are uncontrolled and for something like a horse without a rider/handeler in the scene you have very little control. Thus you need as much time as possible to allow for the horse to give you the shots. You could turn up and be done in 5 mins or a few hours depending on how they are.
2) Learn something of horse posture and posing; horse people tend to be far far more aware of posing in their horses and pictures than most people. At its most simple the ears perked up and facing forward is generally considered between essential and highly desirable - and most other angles are not going to be suitable (and ears right back is a no go). Since you're going for a natural scene you won't have to worry as much about the stance of the horse, however different breeds to have different standing poses that, for a more formal conformation shot should be aheared to (generally the owner/trainer/rider can advise on this)
3) Those dog clickers - a rattle - one person I know even uses a series of random looping horse calls on their phone on an app. All things to draw the horses attention toward yourself (or toward an assistant if you want the horse looking in another direction) - all this adds together to allow you to get those ears forward as when something interests a horse that's what they will often do. Mind though that sometimes you will only have one method work once before they are bored.
4) IF you're going for a natural shot then getting there early lets you go into the field with them and lets them relax and then forget about you. This means that you're more likely to get natural posed shots and also interaction (eg mutual grooming) between the horses in the field.
5) Reflectors are good; just make sure you open them up well out of sight of the horse (those reflectors popping into view will typically be a shock). It's also advisable, where possible, to bring the horse to the reflector rather than the other way around so that they can get used to it. You will want an assistant for this (though I'm sure you know that already)
6) Talk to and listen to the owner/trainer/rider and make sure they are with you when dealing with the horses. They know their horses and can give guidance on what will and won't work. Also be mindful that if anything worries the owner/trainer/rider that will in turn worry the horse as the pick up on that. So if they are not happy with using a reflector or such then be aware that even if the horse has no fear of it they might pick up on that worry from the owner/trainer/rider
Note you mention using 24mm - I assume that you're going for a landscape shot with the horses forming a small part of the scene in this case. In that case you might not need the attention grabbers, however you will need all the time in the world if you want them in a good position (could be you roll up and the first half hour you have to coax them to face the right way so that they are not just showing their butts to the camera)
If not then you'd ideally want to switch to a telephoto lens. If you're shooting closer then with a horse perspective distortion is a big thing as they are very large subjects with a lot of depth to their features. 50mm at the very least but 100mm or more is more ideal. That way you're far enough back when framing that you're not going to get noses bigger than heads and such.
I will be bringing my 70-200 with me so no problems with lenses! Thank you!
