'You ready?' James Potter's eyeglasses reflected torchlight as he turned to grin at his partner in mischief.  Sirius Black merely smirked, grabbed the edge of James's Invisibility Cloak and pulled it around him.  James adjusted the Cloak to finish the job and thus they made their way to one of the lesser-used stairways leading out of Hogwarts.

Memories rather than moonlight helped them cross to the Forbidden Forest -- only a sliver of the setting crescent moon showed above the mountaintops.  James and Sirius made sure a line of trees and bushes lay between them and the Gamekeeper's hut before unCloaking and incanting Lumos.

'How far do you want to go?' Sirius grinned eagerly.

'Just far enough so we have some room.  If you have to haul my charm-damaged body back I don't want it to be too hard for you, right?'  James pushed some branches out of the way.  He grimaced and shook spiderweb off his hand.  'Euch.'

Sirius snorted.  'Mate, we have this charm down flat, you know that.  All that's left is the actual test.  Peter'll catch us up later, if he can be bothered to apply himself instead of whining.'

In short order, they broke into a small clearing where starlight cast their faint shadows onto the weedy grass.  They stared at each other for a moment; Sirius finally said, 'All right, then,' and put his wand away.  James stepped back and watched intently.  The handsome teenager closed his eyes and screwed up his face, invoking all the different components of the Animagus spell, focussing on what the books said was the most important:  concentrate on what you believed to be your greatest strength.

Nobody knew what form they would take until they actually performed the complex charm for the first time.  The boys had read horror story after horror story about wizards who had failed, ending up half-beast/half-human; or who had achieved an animal form without retaining their human minds and had lived out their lives as somebody's pet.  This had scared Peter into wanting to abandon the effort, but his mates wouldn't let him, laying on the guilt about gratitude to Remus for all his tutoring -- but all three knew Peter wouldn't be ready for weeks yet due to his lack of nerve.

Despite their loudly stated confidence in themselves and Sirius's natural Transfiguration talent, James still had tiny twinges of uncertainty.  What if they'd forgotten something?  Suddenly, there was a soft *pop*, and Sirius no longer stood before him.  Instead, James strained his eyes to make out the huge black shape before him.  He pointed his wand-light directly at it; pale eyes glowed back at him like captured stars, and James jumped back with a stifled yell.

'Sirius... you're... you're a Grim!  You're the Padfoot himself!  You did it!  Erm... you're still Sirius, right?'  The legend-sized 'dog' barked once; James thought his friend's tail wagged furiously.  The dog bounded forward, licking James's cheek with one big slurp.  'Aaauugh, stop that, you prat!'

Another *pop* and Sirius stood next to James again, beaming to split his face.

'It was incredible!  Come on, Potter, you try it!'

James gathered himself as his best friend had just done, reached deep inside himself and his magic... and felt his reality shift.  Next he knew, he stared down at Sirius's astonished face... and his head felt... heavy.

'James -- you're a stag!'  Sirius bent and grabbed one of his friend's thin legs, pulling it straight so James could dip his head and see his hoof.  He shook his head experimentally and felt what must be a full rack of antlers tug at his skull.  Sirius couldn't contain himself -- he jumped around in glee before holding still long enough to change again.  The massive black dog bounced down into a play-bow, barking at the stag to come play -- then bolted out of the clearing.

They took turns chasing and being chased, revelling in their new forms.  James had tagged Sirius out and was now the chasee when a pale form bounded without warning right into the stag-boy's path.  James avoided it with difficulty but -- not used to his new long legs -- sprawled over a fallen log.  Sirius, pelting after him at full tilt, tripped over his friend in turn, which left both boys on the ground staring up at their roadblock.  Startled, they lost their control on their charms and shapeshifted back to human.

A centaur.  Young, from what they could see with their animal vision before losing that advantage -- faint light shining off him because he was blond all over, from top of human head to the end of palomino tail with matching Nordic-pink skin.  He too had started at the unusual spectacle of a mythic dog and a stag playing tag.

The boys stayed frozen in alarm.  What would he do to them?

Stare, apparently.  The centaur finally stepped forward, leaning down to get a better look at them.

'I am Firenze.  And you were not what you appeared to be.  Humans... be warned.  Mars is rising, and Saturn is ill-positioned for your ephemeral affairs.  Your people would be well-served to look within yourselves for a solution to your problems.  And you would be well-served to leave the Forest now -- the Acromantulæ are restless tonight, and you are unused to those alternate forms of yours.'

Firenze straightened, turned... and bounded off, swallowed by the shadows in seconds.  James and Sirius looked at each other, eyes wide, and bolted away after changing back to creature-form.  They ran neck-and-neck all the way back to the edge of the trees.

However, by the time they'd reached the castle and re-Cloaked, they could barely suppress their triumphant snickering enough to stay concealed.  They had done it!

Still... once safe in Gryffindor Tower, James mused out loud, 'Wonder what that Centaur was going on about, looking within ourselves....'

Sirius shrugged. 'Who cares? We DID it, James! We're going to own the Forest from now on!'