Looks like we have some alien horses to deal with.
Though I wonder, would a six-legged horse be faster than a four legged one?
Broadly speaking, no. Six legs are good for either having redundant legs if one gets torn off (which benefits insects) or spreading weight of a massive frame across more limbs. It takes far more energy to move your legs than the rest of your body because your legs have to not only be moving with the inertia of the rest of your body, but swinging back and forth, as well. Having more legs means you're spending more energy just to have those extra legs keep up with the ones you already had, so you're directly cutting down on stamina. You'd only get an advantage if you're so heavy that square-cube law starts making your legs start having to have ridiculous girth to power both themselves and support the body above.