I was on route 12 filling up my dex and I encountered a taurous horde. It said A taurous horde appeared. In the horde was a frickin miltank I sont know how this happened so I would like an explantion
Sometimes, hordes of different kinds of Pokemon appear. Taurous and Miltank are actually male and female forms of each other, so I suppose this makes sense.
Another horde with two types of Pokemon in is Durant and Heatmor hordes. They are natural enemies, so they attack each other instead of your Pokemon! Its quite funny if you find one of these hordes.
So no, this is not a glitch. I hope I helped!
Source and Source
its not a "rare chance" though. a tauros horde will always have a miltank
A Tauros horde on Route 12 will always include one Miltank.
It's not a glitch, you have a rare chance of finding a Tauros horde, Tauros hordes always have a Miltank as the 4th Pokemon in a horde in route 12. (other routes have that aswell, e.g: Route 20 a horde of Trevenant with a Sudowoodo)
Source (you see how under "hordes it says Miltank, rare with tauros? that's what it means.)