Fort Donelson does not have much in the way of monuments. In fact I can think of only two monuments in the park. Considering how this battle helped decide the fate of the war it seems odd that it lacks a monument to Grant's army.
The oldest monument is this one to the Confederate soldier. I've read that the monument rests on top of a mass grave. I've also read that the mass grave is just near here and the NPS is not sure where it is exactly.