@thps48  
or maybe Spike read the spell on a book and took it to Rarity…and they didn’t read about the side effects.

Beings that aren’t unicorn ponies can magic in-show with the help of potions (Cutie Pox, Hearts and Hooves), so why can’t Inspiration Manifestation be a pot?
 
Also, I’d love to see a Quest of the Princess Ponies reference eventually, not just by involving six princesses, but also giving somepony a magic wand. I’m aware G3 also had magic wands, so, if it’d reference that variant, that’s cool, too. :j

Oh snap

@PonyPon
 
I believe, and this could entirely be speculation, but it might be likely that either rarity was having a problem getting her ideas from her head onto paper or in her fashion form. Or, the spell wasn’t clear in what it did or the side effects it could have, and they didn’t know the spell would giver her idea manifestation powers.
 
Until the episode airs, the question will have to be answered speculatively.

And Rarity Evolves Into… Missingno, apparently!

@Fortune  
okay, you know what? Not the point.
 
What does that extraordinary ability even have to do with inspiration if the problem is that she can’t think of anything?
 
 
@GLARGH  
I think it’s going to be another spell you activate by reading it.

I’m hoping we have a Rarity version of Secret of My Excess.

@PonyPon  
It’s not out of place alongside spells that switch destiny, cause things to want to eat non-edible objects, the good old “Want It-Need It Spell”, and the recipe for a Love Poison being readily accessible in a history book.

@PonyPon  
The better question is this.
 
Since when could Spike cast spells on anyone?
 
Do Dragons have special Dragon Magic?
 
Is that why Rarity’s eyes and magic are green like Spike’s fire? Because he filled her with his mystical Draconic essence?

@FSin  
So…she’s going green lanturn again? Kinda?

@FSin  
seems like the spell is supposed to inspire her. guses it makes her a little crazy.

@FSin
 
Why the hell would that kind of spell even exists?

Prolly not changelings but I can hope…
 
If it is I would prefer if it normal changelings not Chrysalis herself.

The episode’s synopsis says that Spike casts a spell on Rarity, making anything she thinks about actually come to life. So I guess a side effect is that it makes you mad with power

oh my god

If only.  
(Best Royalty impersonating Best Pony? Hells yes!)