I am not really sure, but I think that cel came from it came
from the episode "Tenticool and Tenticruel". It is probably
the part where the Pokemon go off to fight the giant 
Tenticruel and Pikachu jumps on Pidgeotto's back. That is 
episode number 18. I can't think of another episode where
Pikachu is on Pidgeotto's back, then again I haven't 
watched Pokemon in a while. Anyways, I hope that helps!