I don't own Harry Potter if I did, Sirius would still be alive.
Severus Snape is an animagus. His animagus form is naturally a bat.
Severus flew through the window in his potions office. He had just finished his midnight flight with his fellow bats. The potions master circled his office, (he was only a tiny bat after all) before crash landing into a bucket of water.
Snape could feel the cold water spill and cursed himself for being such an idiot. The water was spilt all over the floor and the bucket laid on its side. Snape decided that he could clean up the mess later in the day.
He hopped up and flapped his leathery bat wings. That was when he heard a miaowing sound coming from outside past his locked door. Snape understood at once that it was Mrs Norris the cat which belonged to Argus Filch.
The tiny bat began to panic as he realised that the miserable cat would follow him not fully knowing who Snape was. He was an unregistered animagus and nobody, even Dumbledore, knew that he, Snape, had the animagus form of a bat.
Mrs Norris clawed at the door and Snape glided to hide behind his desk. He still wanted to enjoy his animagus form while he had the chance.
Just as Snape dreaded, Argus Filch had been alerted and the old caretaker shuffled towards Snape's office door and tried to pick the lock using an old hairpin. A click echoed through the night air and the door opened.
Snape was shocked, his door was immune to the spell, alohomora, but it was unlocked by a simple hairpin. He made a mental note to himself to fix that lock of his when his thoughts were interrupted by Filch's raspy tone:
"This is Snape's office, Mrs Norris, there ain't no one in here."
Mrs Norris growled but Filch had already left. She entered the room, her eyes detecting the slightest of any movement. Snape waited behind his desk, he wished the dreaded cat the leave the room and give him some peace.
But the cat had no intention of leaving and just to Snape's luck, Mrs Norris saw the bat's reflection in the water spilt earlier. Swiftly she pounced up and locked Snape in her jaws.
Snape was caught in Mrs Norris's mouth!
Snape soon regretted his decision to stay as a bat as he struggled against Mrs Norris's tight grip. The more he wriggled about, the harder the Mrs Norris held him. It was excruciatingly painful. Snape could feel the sharp feline teeth which dug into his fur.
There was nothing he could do but wait for Mrs Norris to put him down. Snape could feel the strength leaving him. There was no way he could transform back into a human with the energy he got inside him which was draining rapidly due to his efforts to free himself.
Mrs Norris on the other hand was quite eager to show her master at what she had found. She ran towards Filch and rubbed against his ankle to get his attention.
"What is it sweetie?" Filch asked. Meow! was the reply while Mrs Norris dropped the bat onto the floor in front of Filch. "Well, well, what have we got here?"Filch murmured.
Seizing this moment, Snape mustered up all the strength he had and transformed back into his human form and fortunately was back in his clothes. He whipped out his wind and shouted, "Obliviate!" and pointed his wand at Filch. Filch staggered back but Snape was already striding back to his bed.
Snape had enough for one night and once his reached his bed, he threw himself under the covers promising himself that he will fix his door lock, heal his wounds from Mrs Norris's bites, and aim a good kick at the cat. He'll also have to clean up his bucket of water.
It would be a long time before Snape turns himself into a bat again.
I'd just thought it might be cool if Snape was an animagus and that he was captured by Mrs Norris.