Bird v Dog

[gerkin]

Why is an pretty bird called a chick and an ugly bird called a dog ?

[Quizmonster]

On the assumption that you are talking about girls rather than other of nature's creatures...'chick' has been used in this slang way since its first appearance in the 1920s in America. (Prior to that, of course, a chick or chicken might just be a term of endearment used to refer to a child as well as a domestic fowl.)
'Dog' has been used as a term of contempt since the 1300s. Again, it was originally used in today's sense to mean something inferior or not up to scratch...eg ugly...in America since the 1930s.