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USM is VERY unlikely to contain a full roster of following Pokemon, and here's why.
The walking animations that have been datamined for all Pokemon are for the high poly Pokemon models. They are linked to its skeleton and rig. High poly asset are a heavy hit on performance- like you see in double battles in SM.
In order for the overworld to not have poor performance, Game Freak created low poly versions of some Pokemon and trainers . Compare your trainer model used during a battle to the one used during overworld exploration.
People who believe that because there are the animations for walking means that following is happening don't understand that Game Freak can't use the high poly animated models in the overworld. That would likely destroy performance. They would have to create a completely different low poly model for each Pokemon, and map the animations to that. Even if the low poly models were created using the same rig and skeleton as the high polys, animations are hard to get working flawlessly across multiple models without clipping problems.
The way I see it, there are only four possibilities this trailer snippet suggests.
1. Game Freak somehow managed to get the high poly models to run well in the overworld, and they will simply use those. (If this is the case, it makes you question why low poly models were ever used in the first place).
2. Game Freak has created a whole set of low poly Pokemon models that have the same rig as the high poly, and will transfer over the datamined walking animations we have seen to the low poly (This is a large amount of work by Game Freak standards).
3. The number of following Pokemon will be limited to ones that already have low poly models that you see in the overworld in SM (such as the starters, Miltank, Pyukumuku, Pikachu, Tauros etc.) because they have already done that work.
4. Rockruff will be a unique case and be the only following Pokemon (or NPC).
