mouse

I assume that your question refers to why you call the mouse on a computer, a mouse rather then a rat. Why, while mice are not really popular animals, there are some cute ones = you remember Pixie and Dixie. Rats on the other hand almost always have a negative connotation - I mean there was Ben - Michael Jackson sang a love song to him - that should have been a sign. I hope this was your question.

I have no idea... because it sounds better perhaps? But in portuguese we use in mouse (the animal) and mouse (from the computer) the same word, it doesn't have a difference.

Because it looks like a small mouse, instead of a kitten.

why cat and not puss