Where do Patronuses come from?

A series of explosions could be heard from the door. Somebody was hitting the mahogany boards so hard that the dust was swirling around in the air.

Snape cocked his eyebrow. He had sent for Hagrid, true, but he didn't expect the gamekeeper to arrive so quickly. Finding six hundred and seven hairs from the Acromantula's leg should have taken him much more time.

"Alohomora!" Potions Master waved his wand and Hagrid, who had been aiming to hit the door once again, almost fell down on the doorstep.

"Professor Snape" panted the giant and leaned against the door. "Blimey, I dunno how to tell ya, but..." Hagrid was lost for words.

"What's going on, Hagrid? Did you fail to meet my Patronus?" Snape asked dryly, but the gamekeeper shook his head.

"That's the problem, sir. You'd better take a look, the poor thing came after me..."

Snape paled and bolted into the corridor. One look was enough for the desire to murder to build up in his veins.

In the corridor, lit by the light of the torches, stood his Patronus – a silvery doe. A small, silvery fawn stood at her side.

The Hogwart's castle shook from his scream.

"Poooooootttttteeeeeeer!!!"