@Maximus Prime 01  
Like a whore. “Easy to sleep with.”

What does she mean by “Makes me feel easy”?

@Ferrotter  
It wasn’t a conscious decision, since Rainbow obviously wasn’t listening to the offer, since she was more focused on getting the book. She immediately tried to call the whole thing off as soon as she actually comprehended what she agreed to.
 
Even if you ignore that, can you really call that slavery? She wasn’t abducted or threatened to be beaten or killed. It was a barter, the entire point of the episode. She gets the book if Fluttershy helps train the Orthros. Fluttershy was willing to go along with it because it would make her friend happy. Even though she was understandably reluctant, she still made her own choice about whether to go or not.
 
So all in all, I would hardly call it slavery. She was just going to be a live-in trainer for a period of time.
 
I guess most people take it that way simply because of the time it would take. I just went to do a fact check, and Fluttershy says she doesn’t know how many moons it would take to train it. So it may take a year or more. If the timespan was, say, a week for training one, I doubt the majority of people would be complaining about it as much.

Working to get payment still isn’t slavery.

What almost? Taking it back after doesn’t mean you didn’t do it.