@kokl123  
Hah, exactly what I was talking about :p

what if there’s no minion that’s equal to the mana?

Not bad actually. Its a good fallback/emergency card, but feels decently balanced by the fact that its random.
 
 
That being said, there should be an explanation for what happens if you have NO cards matching that mana cost (like if you play it while at 5 but have no 5 mana cost creature).  
The logical result would be nothing, but I kind of feel there should be a joke character that shows up (like a 1/1 creature) with some funny flavortext, like the worthless imp card.

This would literally be the best card ever printed.