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.
The cat is Snowball, and there are several Snowballs. The dog is Santa's Little Helper. Snowball and Santa's Little Helper. Snowball (cat) and Santa's Little Helper (dog), I think. Cat: Snowball, I do not know why. Dog: little helper of Santa (it was a racing galgo that lost a race making Homer losing his bet). As Homer was broken and found the dog in the street it was the only present he could offer to his sons. Santa's Little Helper is the dog.