There are loads of equestrian areas communtable to Munich. People have horses here like people in the States have golf memberships - they're everywhere. For example, I live in
Unterschleissheim (>20 minutes to Munich center) and there are at least 3 stables on my street and a few other people who keep horses (their own) on their own property.
My colleague actually has 8 horses in her stable (4 of her own, 4 belonging to other people) and takes care of them (with help) while still working 32 hours a week at my company. And she just rents this house and stable from someone.
She told me to look for a magazine that you can get anywhere called 'Bayern's Pferde Sucht and Sport' (Bavaria's Horse Breeding and Sport). She said it can be found anywhere that magazines are sold and that there is an advertising section in the back with lots of people trying to rent out houses with barns/stables etc. And it is especially for the Munich area.