The mouse that roared

I gave my very low level students a big crossword puzzle yesterday. Most of the clues were easy. The students came close to completing the puzzle, using their dictionaries and helping each other. But there was one word that almost everybody got wrong.

"These animals look like very large mice," said the clue.

The word had four letters, and everybody had the first letter, which was R. Almost every student in the class wrote the same thing:

RION