What is a paper tiger? Is it an idiom?

[amit]

Test No. incompl/advan-50 "Papers", question 4

After the first revolution the dictator no longer had much control and became just a paper ..........

(a) lion
(b) tiger
(c) elephant
(d) cat

Test No. incompl/advan-50 "Papers", answer 4

After the first revolution the dictator no longer had much control and became just a paper tiger.

Correct answer: (b) tiger
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why lion cant be used instead of tiger

[Alan]

I'm afraid it's the accepted idiom and there's nothing I can do about it!!
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English as a Second Language
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