Psych's a great show if you haven't seen it. I find the characters are some of the most entertaining I've ever seen. Speaking of characters...
Shawn Spencer: Rainbow Dash (you might think this should be Pinkie, and I might agree except who would be Yang later?)
Gus: Applejack (could also easily be Spike but... eh)
Lassiter: Twilight (yeah, this was a no brainer)
Juliet: Fluttershy (more FlutterDash yay!)
Doctor: Starlight (I just threw her in. No real reason.)
Z: Discord (Odd, would get pegged as being a murder. Honestly, this could have been Trixie though...)
'The Victim': Apple Bloom (Truth be told I forgot all about this character. I just like Apple Bloom so I slapped her in.)