I can imagine a dialogue exchange between these two characters going like this:

(Centaur?): "Genie? What did you do to me?"

Genie: "Well Master, you wished to have half the horse power. So I gave you half a horse's body!"

(Centaur?): "I meant for my Truck Genie!"

Genie: "Well you should have been more clear about that on your wish. The good news though, is that now you won't be needing that old thing anymore! You can gallop as fast as you want! Like a Horse!"

A centaur tf by a wish to a genie? Awesome!