I just want to point out that while alexa having Gogoat may decrease the chance that Ash gets one, it only slightly reduces the chance that he'll catch a Skiddo. Ash might get one that will stay a Skiddo and not evolve into Gogoat.
But it seems like Serena also would fit with a Skkido/Gogoat as she wants to race. Two similar pokes in the main cast would be boring.
"Two similar pokes in the main cast would be boring" indeed. the creators wisely avoid that. but I have yet to here any thing that says the racing is not exclusive to Rhyhorn. she get Rhyhorn for racing and leave Ash to capture Skiddo.
That is one possibility. But if races consisted of only Ryhorn, then that would be boring too. So Skkido/gogoat is not for Ash I think. Just go with Phantump.
they did call it ryhorn racing which means only ryhorn
i want ash to catch a noibat and riolu the rest i dont care