Cățeluș is the diminutive form of cățel. This form expresses littleness, cuteness, or fondness.
Why is it 'cățeluș' and not 'cățel'?
Yes, I have the same question.
im guessing its like saying "doggy" in English but im not sure
Or i could say why the hell would you want to knock a puppy down in first instance?
Indeed, I was wondering the same thing. In the Italian course we learn that all the animals are dead; here we learn how it happened.
Could one also say cățelica as a diminutive?
No. In this case the diminutives are "cățeluș" and "cățelușă" for male / female