I think they should.

Why would it appear? So it can farm views or something?

^ Precisely

It can do that even if it doesn't appear in horizons. Greninja is greninja, it will always stay popular among people. They'd rather give the majority what they want.

Still, it makes little to no sense why Greninja would appear. Tbf they added shiny Rayquaza and Galarian Moltres, so I can kind of see that.

Nah I don't think so....