You are a teenage young man.
"What...nothing's happened." You paused and looked up at the apprentice and were shocked to see the form before you. Instead of the freckled young girl from before there was a massive scaled form slumped on the floor. You approached cautiously, tip toeing as it shifted and you blushed as you realized you were looking at its ass, covered in pale green scales. Definitely female, and huge, even bent over like this as tall as you. It'd be giant when it stood up.
She groaned out and muttered, "Did...did it work?"
"Uhhhhhhh..." You were never that clever at times of pressure and you backed away as she stood up, head brushing the tree branches.
"What the..." She looked down at her hands and then let out a scream that nearly threw you off of your feet as she looked down at her claws. The apprentice who'd summoned you was a twenty foot tall anthro female dragoness. Pale green, nude, and panicking. Her wings lashed out and you dove for cover as a gale of twigs and leaves were thrown up all around you.
"What did you do!?" She roared as she clawed at the trees around her.
"Nothing! Nothing!" You'd found a boulder, and were looking for something more permanent only to have it hauled out from behind you, putting you face to face with the enraged dragoness, holding three tons of granite overhead and glowering, acid dripping from her mouth onto the forest floor.
"You ruined the ritual! I'll kill you!"
"Wait, wait, wait, wait, wait, wait!" Your hands were up and she paused, still holding the boulder and fuming, muscles standing out on her neck as she strained against invisible bonds and her eyes went wide.
"Oh no...Oh by the Lords of Morning no! I...you...we..."
"What is it?" You had backed away out of easy boulder dropping reach as you watched tears well up at the corners of her eyes.
"I fucked it up! I...I...I did it backwards. I made myself your familiar!"
You blinked and looked up at her, "Oh...um...sorry?"
She shuddered, still holding the boulder, tears streaming down her muzzle.
"Do you want to sit down and talk? Sans boulder?"
She quivered slightly but nodded, still holding it.
"Alright then...come tell me what's going on."
She set the boulder down then, and gave you a detailed explanation. Wizards summon familiars, who they believe are created from the fabric of the Magic Realm, to have servants that help contain their power, extend their life, and act as transformative tools and sometimes playthings. Despite your protestations she refused to believe you came from a world of your own, "Every familiar says that but it's been disproved. You're deluded." If you die, so will she, and furthermore she must obey your commands. Supposedly you can change her at will, but when you try to all that happens is that she briefly turns bright red. You stopped trying and sat down.
"So what now?"
"Well...we need to get some time to get me back to normal, otherwise people will know things are wrong. I may have just kept you human to blend in, after all. Then we'll need to go talk to my teacher before..."
"Before what?"
"He'll be able to fix things, I hope." The desperation in her voice was enough that you reached up and gently patted her on the lower back, offering a feeble "There, there."
In the end the two of you decided to take shelter nearby and work on getting her back to normal. You were lucky though, and found a dry shelter nearby in the form of a...