Onions contain a compound which is toxic to the red blood cells of cats. The red blood cells are destroyed and this results in anemia. The specific type of anemia is called 'Heinz body anemia' because the red cells develop an abnormality called a 'Heinz body' which can be seen under the microscope.
Dug, the dog, is easily distracted by ... squirrels (well, he's easily distracted by everything, but especially squirrels). He can be talking (yeah, it's Pixar/Disney so the dog talks) & right in the middle of the sentence, he shouts, Squirrel!. Then he carries on with whatever he's talking about.
Mr. Jingles is the tiny mouse that joins the group on The Green Mile. At first the guards try to kill him, but then everyone becomes rather fond of the friendly little mouse. Eventually Del takes him in as a pet. Mr. Jingles symbolizes the loneliness that the men feel on the Mile. They are even willing to take in a mouse. Only Percy still holds a grudge against the mouse, and one day he steps on it. After Percy steps on Mr. Jingles, John uses his powers to save the tiny mouse.