Yes, that's completely normal; in fact, there are several other occasions where Pokemon of different species will appear in hordes. Bulbapedia lists them:
- A Plusle horde on Route 5 will always include one Minun in Pokémon X, and vice versa in Pokémon Y.
- A Zangoose horde on Route 8 will always include one Seviper in Pokémon X, and vice versa in Pokémon Y.
- A Nidoran♂ horde on Route 11 will always include one Nidoran♀.
- A Roggenrola horde in Reflection Cave may include one Carbink.
- A Tauros horde on Route 12 will always include one Miltank.
- A Durant horde on Route 18 may include one Heatmor.
- A Trevenant horde on Route 20 may include one Sudowoodo.
They seem to be themed in some way; for example, your horde with Trevenant and Sudowoodo is based on trees. Other matches include Plusle and Minun, which are counterparts, and Zangoose and Seviper, who are rivals.