The highest pony that completed the game with the shortest time is Applejack, who stacked the 21 cups in 33 seconds, while the pony that completed the game with the longest time is Strawberry Scoop, stacked the 21 cups the game in 60 seconds (and also the only pony to finish the game in one minute).
There are no pegasus, unicorns, or alicorns because of the following things:
Unicorns and Alicorns use magic, which would be considered cheating, and would be disqualified from the Stack Attack game.
Alicorns and Pegasi ponies have wings, which are also against the rules of the game.
This is the original, which usually features only one person doing the Stack Attack game: