Over a series of trades on Pokemon XY, a trainer traded from a Fletchling caught on Route 2 all the way to a Dialga. Continue, to see the process!
And then, traded for a Pikachu
Subsequently trading for a Floatzel…
Traded for a Gurdurr, which evolved into a Conkeldurr…
Which was traded for a Milotic
Which was exchanged for a Xerneas
And finally, last but not least…A Dialga!