Aunt Gwen seems quite at home on a horse, which she has ridden many times in the Blue Grass regions of Kentucky.
As to Philander, the same does not apply.

He acts as though in deadly fear of being pitched over the animal's head.

The fates decree that the largest horse of all falls to his lot, a raw-boned, loose-jointed specimen of equine growth, and the little professor looks like a monkey perched aloft.
If the beast ever had any martial ardor, it has long ago died out, and yet to the excited fancy of the professor, he might as well be upon the back of a prancing, rearing, snorting war-horse.

When the equine wonder shakes his long ears, Philander imagines he is about to perform some amazing trick, and, filled with a new dread, he clasps his arms around the poor creature's neck, and calls out: "Whoa! there's a good fellow--be quiet now! I wouldn't hurt you, boy! Whoa! I say.

Hang me if I don't believe you've got the devil in you.
Want to kill me, eh?
No, you don't.