A few days ago I had a Horde Battle versus 4 Zangooses and 1 Seviper. I took one of the Zangooses out, but then something weird happened. The three remaining Zangooses started attacking the lone Seviper, taking it out before it can even move. Is there any other Pokemon who fight like this?
No. The only one that does this is Seviper and Zangoose. In some encounters you can find a minun with plusles in a horde encounter.
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:
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!
Yes, I have seen four Durant fight against one Heatmor in a Horde encounter in route 18 in Pokemon Y, maybe in Pokemon X is the other way.