This only happens in two scenarios, and both of these scenarios involve two species of Pokemon that dislike each other. This only happens when there is:
A Zangoose horde with one Seviper or vice versa:
-Zangoose Pokedex Entry (Black 2/White 2): It’s Seviper’s archrival. To threaten those it encounters, it fans out the claws on its front paws.
-Seviper Pokedex Entry (Black 2/White 2): Constant polishing makes the edge of the blade on its tail extremely sharp. It’s Zangoose’s archrival.
A Heatmor horde with one Durant or vice versa:
-Heatmor Pokedex Entry (White): Using their very hot, flame-covered tongues, they burn through Durant’s steel bodies and consume their insides.
-Durant Pokedex Entry (Black 2/White 2): Individuals each play different roles in driving Heatmor, their natural predator, away from their colony.
Note: Some Pokemon may also be found in a horde of another species, but the only time where they will fight each other is in the above scenarios.
Hope I helped!