Overrun while mounted, who makes the check?

[Etruis]

Now, one of my players is having a stroke over this specific problem and I need help on determining it myself, a lot of people have said it was one or the other but I haven't seen any real evidence of the correct answer, this problem came about when the feat Mounted Onslaught and I would really prefer if this was just done and over.

Thanks for the help if I can get any.

[wraithstrike]

You make the check. This feat just builds upon the trample feat which specifies that you make the check.