Fluttershy is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Defender, flying , : Put a +1/+1 counter on each creature with a tail target player controls. Stare down up to one target creature until end of turn. (It can't attack or block as long as you're looking directly at it.)

How it works : Fluttershy

Original English text — Wizards of the Coast / Scryfall

No additional ruling has been published for this card.

Interactions with other cards

These cards' text mentions Fluttershy.

Princess Twilight Sparkle

Flying Other Alicorns, Horses, Pegasi, Ponies, and Unicorns you control get +1/+1. {W}{U}{B}{R}{G}: If you control Applejack, Fluttershy, Pinkie Pie, Rainbow Dash, and Rarity, everypony wins the game.