Possibly drunk as a skunk might have related to cockney ryming slang not sure how this fits in with the others though.

I've tried to look this up to no immediate avail but I can only assume it's because those animals might exhibit tendencies that suggest drunkenness. You know, a strange walk, or erratic behavious, or some such but it is only a guess and I'll keep looking.

i guess its that when your drunk ppl act like animals....... as to the animal reference its usually when your drunk you can come out with sayings like that and it sticks

Expressions like this are used to create a larger than than portayal of a situation. The use of metaphors and similies are are part of English and there are countless examples which are used to conjure up a mental image usually to highlight the absurdity of the action.

Could be a coincidence:
Drunk + Skunk rymes.
Newts are surrounded by their drink of choice (as are goldfish - so does that work?)
Rats' arses hang out and are offensive to observe .....QED