This ad is taken from the end of a My Little Pony vhs released by Children's Video Library. The tape is just called My Little Pony but it contains the first TV special from 1984, which is also known as Rescue from Midnight Castle or Firefly's Adventure. In the UK this special was also called My Little Pony in Dreamland. As far as I can tell, the animation is from the Rainbow Pony TV commercial.
I don't know why they would use a TV commercial to talk about another special. It's very confusing and has lead people to think that there might be a lost Rainbow Pony TV special out there. I remember one website awhile back where someone actually claimed to have found the special but as far as I can tell it does not exist. The ad is probably just referring to the second TV special, My Little Pony Escape From Catrina.
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After the success of the original My Little Pony special (which would later be known as Rescue...