Here's why:
* "Simiie" is not a word. The correct spelling is "simile."
* The structure is incorrect. A simile uses the words "like" or "as" to compare two things.
Here's how you could correct the sentence:
* "As fierce as a horse" This uses the correct structure and word choice for a simile.
* "A horse that is as fierce as a tiger" This uses a simile to compare the horse to something else.
Let me know if you have any other questions.