Two different Pokemon species may appear in the same horde. Plusle and Minun is one that does this, others include Seviper and Zangoose, Heatmor and Durant, and Carbink and Rogenrola. There are others besides these, but these are just the examples I can think of right now.
Different Pokemon seem to appear in the same horde when:
1) They are counterparts to each other - Minun and Plusle probably fall under this category, as does Tauros and Miltank
2) They have a long history of feuding with each other - this is the case with Zangoose and Seviper, and Heatmor and Durant. In these cases the Pokemon turn on each other in the battle and attack the opposite species, instead of your Pokemon.
3) They have similar designs - this is probably the case with Carbink and Rogenrola, although you could argue Plusle and Minun could fall in this category as well.
Hope this helped.