Which preposition to use with fenris

at Occurrences 2%

The three pack horses walked in file between, heads low, tails whisking; and Ben, with Fenris at his horse's hoofs, brought up the rear.

atop Occurrences 1%

He saw in one glance that the craft was rather heavily laden, Fenris atop the pile of duffle, and that Ben was paddling with a remarkably fast, easy stroke.

Which preposition to use with  fenris