Roll a d10.
1-3: The character starts looking a bit more like the animal. Some bodily feature becomes more animal-like (cat’s eyes, ears take a slight horse-like shape, barking voice)
4-6: The character gets an impulse to change one type of behaviour to be more like the familiar’s (starts sniffing like a dog when he smells something, becomes aloof like a cat, or starts purring when someone pats his head)
7-8: The character becomes attracted to the sort of habitat of his familiar, and increasingly uncomfortable elsewhere (starts climbing trees, or doesn’t sleep in his bed anymore, but on the floor)
9-0: He changes his diet to that of his familiar (eats raw meat, or no meat. Starts eating insects)
Thomas the wizard has a frog familiar, and frankly, had it for just a bit too long. He is standing perfectly still now, only his bulging eyes move as they follow the fat fly buzzing through the air. Then, with a lightning quick move of his arm, he captures the insect, puts it in his mouth and swallows.