This seems like a simple question -
You need to think about some stuff here. x]
Like who will fly you places, etc. etc.
(Not all of your Pokemon have to have reasons behind them.)

Oh - and no legendaries. -_-;
Did Ash ever catch a legendary?

You can even have some fun with this, like say where you got the Pokemon, etc.

Okay, my team:
- Noctowl: To fly me to towns previously visited. Also to be an aerial scout. (Caught as Hoothoot early in my journey.)
- Miltank: No particular reason. (Given to me by farm-owning grandfather while visiting his farm. Takes place after I rescued Stantler.)
- Absol: " " It's ability is useful, though (sensing disaster.) (Caught shortly after Hoohoot.)
- Arcanine *Starter Pokemon*: No particular reason. (Given to me as Growlithe by my father.)
- Spinda: No particular reason. (Caught before Absol in my journey.)
- Stantler: No particular reason. (Actually, it could carry me on it's back like a deer horse thing. :3) (Rescued from Team Rocket shortly after capturing wild Absol.)

And that would be my main team. ^^