otherwise I have rarity muscles version if you want safe or not safe
 
summary:  
spike and rarity finds a pearl with strange power when they pass in the mine under ponyville.  
as a result of a spike accident swallows the pearl and it emerges below these scales.  
has its spike wake touches the pearl and grows in an instant.  
rarity touches the pearl in its turn and becomes a muscular pony.  
of fear the touch-up and resumes its normal appearance.
 
little idea:  
I had the idea of a small series of visuals where the couple sparity goes in search of magical crystals giving power throughout the world.
 
example:  
rarity finds a jewel that allows it to become a crystal pony but its horn no longer takes its normal form.  
So she hides it under a horn glove that looks like her old horn.