A/N: A response to a prompt on kinkme_merlin.
Arthur stared incredulously at Merlin, standing there in the middle of his chambers, with a tail poking out from under his tunic, and cat ears, of all things, protruding from his head.
"Now what'd you do?" Arthur asked, unclasping his cloak and tossing it on the table as Merlin fidgeted in the middle of the room, blushing furiously, ears and tail twitching in time.
"I, um, was trying to, well, turn into a cat," Merlin said, blush now turning quite a spectacular shade of red. "And, well...I think I mispronounced some of the spell."
Arthur laughed as he sat in his chair, while Merlin scowled, though he still came over to help Arthur out of his riding boots. Which looked just ridiculous with Merlin's tail swishing across the floor and his ears - cat ears - pressed flat against his head in a bizzarely adorable way. Up close, Arthur could see even Merlin's eyes had transformed, the pupils turning to slits against the blue iris.
"You look adorable like this," Arthur said, pulling on his best imitation of Gwen.
"...that was actually rather disturbing," Merlin said, having pulled off both riding boots. As he set them in a corner and came back with Arthur's castle footwear, he added, "Please don't do that again. It was creepy."
Arthur laughed as he slid his feet into the lighter shoes, and reached out behind Merlin's head, stroking down his ear as he tied up the last lace on Arthur's shoe.
His nimble fingers froze as his eyes shut with a look akin to relief crossing his face.
Intrigued, Arthur did it again, with a bit firmer pressure, and Merlin's expression took on one of pleasure, almost how he looked when Arthur sucked him off on lazy after-sex mornings. This could be useful...
He rubbed at the base of Merlin's ear, and the idiot actually leaned into the touch, pressing his hear against Arthur's hand like a cat. When Arthur reached out with his other hand to do the same to Merlin's other ear, the boy actually purred in pleasure.
Now, Arthur grinned. He absolutely loved this spell. "Hm...I think I rather like you, this way...maybe get you a nice bedding by the fireplace...and a collar to go with it."
At the mention of a collar, Merlin's eyes shot up, pupils dilated wide open.
Arthur was pretty sure Merlin loved the spell, too.