That area of Proud Lake has always been a multi-use trail system. It's open to hunters, snowmobilers, hikers, bikers and equestrians, etc. As TC. I have worked with and become good friends with many of the equestrians. Both groups play a role in maintaing the trail system. Most equestrians and mountain bikers have no problem sharing the trails. There are always a few exceptions on both sides of course.

It really comes down to sharing your toys and playing well with others. If you can't be respectful, civil and deal with each other's sport on the rare occasions that you cross paths, then don't ride the trails.

Of course you would then be missing out on some sweet old school trails.
I worked on a trail seperation plan with the local Proud Lake equestrian group last year. Unfortunately, the powers that be within the DNR didn't agree with the plan and nixed it. So until or if that ever changes, we share and enjoy the trails.
The only exception is the Kensington to Proud Lake connector, which is on Metro Park land, west of Old Plank Road to Milford Road (which is a great connector by the way). The Metro-Park has seperated Mt. Bike/Hike and Equestrian trails.
See Ya Bill and Sam the Trail Dog
Proud Lake and Milford TC.