You exit open the door and look at the three hallways. Even if you hadn't already made up your mind to try becoming a little more donkeyish, the wonderful aroma wafting from the left would have attracted you that way anyway.
Plodding along on your new hooves, you follow the left hallway down to a large door that opens as you approach. A rumpled-looking young man in a lab coat smiles as you enter what appears to be a large stable. "Come on in! I figured you would head this way. Most all of our subjects do."
He asks you to step on some scales and writes down your weight (385!). Then he checks your teeth and looks you over as if you were a real animal. "Excellent! The process seems to be working well - you are young and healthy. So, you want to become more donkey?"
You find that your voice is now a bray, and nod your head.
He points to the stalls to the right. "There are some Jennys in heat down there. Mount one and you will increase your donkey percentage, but still be humanoid. Mount two, and you'll end up mostly donkey, but with most of your human intelligence. More than two, and you will end up a total jackass - you'll remember what and who you were, but you'll think like a normal animal."
He helps you strip, and then leads you to the first stall. From there, it is up to you.