In , fight is a noun and cat is an adjective describing what kind of fight:
 cat fight
In , fighting could be an adjective or a verb.
[2a] They are fighting cats. <adjective>
[2b] They are fighting cats. <verb>
If fighting is an adjective, it describes what kind of cats;i.e., they are cats that fight to win. If fighting is a verb, or rather part of a verb, then it tells us; i.e. They are fighting/battling cats (or lions).
Note, the phrase cat fight is an idiom. Click here: catfight: Definition and Much More from Answers.com
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