Okay this one is a bit lame but here you go...
Knock knock
whos there ?
Goat
Goat who ?
Goat tee thats who
;p
Haha, someone really likes goats jk.
Under the theme of goats:
Knock Knock!
Who's there?
Goat.
Goat who?
Goat to the door and find out!
Some random ones which aren't too bad

Knock Knock!
Who's there?
Repeat.
Repeat who?
Who Who Who!
Knock Knock!
Who's there?
Nobel.
Nobel who?
Nobel, that's why I knocked!
Knock Knock!
Who's there?
Radio.
Radio Who ?
Radio not, here I come!
Knock Knock!
Who's there?
Waiter.
Waiter who?
Waiter minute while I tie my shoelaces!