This was because it was a prehensile foot, more like a hand than a foot.

The great toe, instead of being in line with the other toes, opposed them, like a thumb, and its opposition to the other toes was what enabled him to get a grip with his foot.

This was why he could not walk on the flat of his foot.
But his appearance was no more unusual than the manner of his coming, there to my mother and me as we perched above the angry wild pigs.

He came through the trees, leaping from limb to limb and from tree to tree; and he came swiftly.

I can see him now, in my wake-a-day life, as I write this, swinging along through the trees, a four-handed, hairy creature, howling with rage, pausing now and again to beat his chest with his clenched fist, leaping ten-and-fifteen-foot gaps, catching a branch with one hand and swinging on across another gap to catch with his other hand and go on, never hesitating, never at a loss as to how to proceed on his arboreal way.
And as I watched him I felt in my own being, in my very muscles themselves, the surge and thrill of desire to go leaping from bough to bough; and I felt also the guarantee of the latent power in that being and in those muscles of mine.