((Obvious answer is monoflying, but I don't think Venti would box himself into one type in such a way. Other obvious answer is all music-based mons, and while this is more along his lines, idk if it'd be ALL music mons. So I've got this.

Kricketune (token music based mon- Plus he would get an absolute KICK out of its cry), Eldegoss (dandelions are the biggest association I have with mondstadt), Dudunsparce (the dunsparce line is based off the tsuchinoko- a Japanese cryptid known for being constantly drunk and also constantly lying- both things I SUPER associate with our dear bard), Decidueye (quintessential archer pokemon, since Venti's weapon is a bow), and I think his ace would be Dragonite (Dvalin!)

Honorable mentions for Pyroar, Mightyena, and Talonflame (the other four winds of Mondstadt), as rotations for his last slot.))