Bubble bursting in-bound.
Observe Gogoat's design. It is supposed to be streamlined to be ridable. The size and shape, the saddle that gets applied, and Gogoat's horns perfectly curving up to reach the main character's hands.
In short, not only is Gogoat's design tailored for ridability, but the riding animation itself is tailored for Gogoat specifically. If they add more ridable Pokemon, everything would need to be reworked for each rideable Pokemon. The player character's animation would need to be done from scratch, and the way the saddle conforms to the body would need to be redone.
Now consider this. Corocoro released info about all four of our new Pokemon, including name, type, one attack and a special piece of flavour text. Pancham's was "has a trademark leaf," Fletchling "has a beautiful cry," Helioptile's was "converts sunlight into electricity. Gogoats? "Can be ridden in the city." Now, if riding were a general feature, it would be very odd to make that Gogoat's info. Every other entry is a unique feature of that Pokemon; it's safe to assume that Gogoat's is as well.
Now if this is true and Gogoat is the only Pokemon that is ridable, that would pretty much destroy any hope of riding actually being useful. Some of you seem to think it would be equivalent to a HM or a bike, but this would just make it necessary to carry a Gogoat on your team. Gogoat would become way too centralised if riding was anything near as useful as a bike. Hell, I'm even ready to admit that there might be a handful of ridable Pokemon other than Gogoat, but the animation problems I've already explained will mean it would oly be a handful. Nowhere near the kind of coverage you would need for this to be a HM-style feature.
Gogoat will be the only ridable Pokemon, and it will only be ridable in the city. It will be Gogoat's gimmick. It will also likely be some kind of throwaway minigame.
