Okay. Here are the Doctor Who companions as their MLP counterparts, the background ponies I assigned to them.
First row: Rose (Derpy/Ditzy), Mickey the idiot (Caramel)
Second row: Donna (Carrot Top), Martha (Colgate/Minuette)
Third row: Amy (Cloudkicker), Rory (Colton Grapevines III)
Why no Captain Jack Harkness? Because he deserves his own pony. End of story.