This happens all  the time, when we are learning a language is similar when we are kids and are learning our language as well. When we are not used to some expression our brain gets a "replacement" or something close in meaning to fill the gap. That is used by some authors of English books (and textbooks).

And the reasoning is also used with funny cartoons as well as in:

http://cdn.quotesgram.com/small/39/6/849124519-c69b690f3b75f3852c1a3df53d467f5e.jpg

"If money doesn´t grow in trees, then why do banks have branches?"

Anyway "crocodile cow" would be tricky, I must admit!