<ahem> Being as this is posted in the Eventing forum and all, I’ll just mention that even highly-reactive OTTB’s and other eventing-type horses can get over cattle. Here in Texas some of the eventing venues have cattle. Cattle may be just across the fence (how 'bout some longhorns, y’all). The field in which one is riding, or even the arena, may reek of cattle that recently occupied it. One popular schooling facility keeps cattle on the cross-country course and doesn’t move them for anything (no other place to put them). The cattle may or may not avoid the riders, but it’s expected that anyone is capable of shooing them away as necessary.
In the rural areas it seems that a day without cattle somewhere in it isn’t really a day, even if it’s just driving by them in pasture. And there are plenty of horses that come to this area without having met cows before.
Some good suggestions were posted above, especially the buddy horse.
Here’s an idea that may work for you. This can be done from the ground or while riding. Begin when you and your horse are close to cattle but at enough of a distance that you can still ride and/or handle your frightened horse. Make especially sure that you don’t allow the horse to fixate solely on the cattle by controlling his/her head. Put the horse to work as much as possible, even at a walk if you don’t feel comfortable going faster, and do your best to keep the horse’s attention on you or returning to you. Large and small figure 8’s, changes of gait, leg yields, etc, using as much space as you need. Do this for several minutes. Then, when you are in the part of your space that is the closest to the cattle, let the horse walk on as long a rein as you can give (again, no fixating). Encourage as much relaxation as you can get, and reward the slightest improvement. If the horse is agitated or spooky, they go back to work. If they are calm(ish) they get to keep walking with light contact or no contact as near to the cattle as they will. Work when spooky. Work when further away from the cattle. Rest when calm(er). Rest when closer to the cattle (even if it’s not really close).
Rinse & repeat; go back and forth between work and rest. As you get a better and more consistent response, end the session before the horse tires too much to maintain the improvement, so that the good behavior will be what the horse remembers most. From one session to the next, keep getting closer to the cattle. It would be good to keep enough distance not to agitate the cattle as the cattle-owner won’t appreciate that.
It may take several sessions, but this should help and maybe have them over the cattle altogether. The goal is that even if the horse isn’t completely calm about the cattle, the horse will remain attentive to the rider and rideable. The cattle won’t be a big deal.
Be sure that you (or whatever rider) does not get tense about the cattle yourself, as that will magnify the negative reaction. When the horse is agitated, practice some deep breathing and calming thoughts, and don’t clamp with either your legs or the reins. It’s important to stay aware of not reinforcing the horse’s sense of alarm.
Actually, if you went to a show, clinic or schooling session and there was something on the grounds that your horse spooked at whenever you went past it, just do whatever you would do about that. It may take more work to overcome the cattle, but the principals are all the same.
The first time my last horse (TB) experienced cattle was when we were riding with a small group. The small herd of cows was loose in the field and wandered nearby from time to time. The other horses were used to cattle and did not react at all. My horse had one big startled spook. No other horse even looked around at them. My horse took all that in and decided it wasn’t worth his trouble and energy to continue to spook. 
You could look on it as a great horsemanship experience for you and your horse, and also a good source of some funny stories! Good luck!
