I've been watching a lot of Invader Zim lately.
Also, I do imagine Pyramid pony with a voice like Gir's, but way more lower pitched and a slight southern accent. I don't really know about Slendy's voice though.
This was done with them as ponies because I can't imagine them doing this as normal dudes.
The sound bites were taken from Invader Zim episodes 'Girl who cried gnome' and 'Planet Jackers'.
This animation is kind of choppy because the only thing I had to animate that could add audio was Windows Live Movie maker. I don't have flash, I only have a video converter :I And I edited the subtitles to say 'PH' instead of 'Gir' (Says I, captain obvious)
Also, Slendy's face inspired by Rainbow Dash
Also i love how the animation came out og PH grabbing Slendy's face.
Invader Zim - Nikelodeon
Silent hill - Konami
Slenderman - Creepypasta
My little Pony - Hasbro