The animals looked like horses and behaved like horses, so why couldn't they be tamed and ridden like horses? Even better, because they were native to the continent, zebras would be immune to many of the diseases and pests that had been plaguing the colonizers' horses.

Why do we not ride zebras?

Anyway, aside from all that, zebras are simply too small to ride! Zebras backs aren't evolved to allow a human to ride a long, let alone to carry cargo or even saddle them. so, even if zebras were the nicest animals on the planet, we'd just cause them pain by riding them.

What makes an animal Tamable?

A tame animal is an animal that is relatively tolerant of human presence. Tameness may arise naturally (as in the case, for example, of island tameness) or due to the deliberate, human-directed process of training an animal against its initially wild or natural instincts to avoid or attack humans.