Here's as close as I can come to your open ended question...
Seasonal rentals are ok if the location is desirable, the property is desirable and the price of the lease is desirable. You have have the utilities available which means that you are paying to maintain those when it's not rented or leased for 3-4 months.
Horses will need a barn or shade and plenty of water. Do you have that included with the property you are interested in? Is it hauled water or city water?
I prefer year leases and so you know, most of the tenants leasing will be showing foreclosures on their credit report. It's just a sign of the times. those that still have horses and are foreclosed on should be considered high risk. Horses are a luxury and should be the 1st thing to go before loosing their home.
Also, are you going to be able to manage the property on your own or hiring a company to do that.? That is also an additional cost sometimes people forget.
The tenants may have issues with fixtures or a door or a window or whatever. Are you going to be able to show up to fix that in a timely manner?
Have you actually looked to see if other horse property is being leased and for how much?
To know your question about how much, this takes more time than a magic wand and you need more than a guess. I would work with a realtor you trust and have them find you rental comparables after they take a good view of the property.
Best of Luck To You,
Keller Williams Realty,