@FanOfMostEverything  
For what it’s worth, on further thought I think it might be… prudent… to take the factual existence of the stuff Discord conjures up with a grain of salt or two. I still think that him pulling the twittermites from Apple Bloom’s subconscious is a bit unlikely, but there’s nothing saying that – to make up an example at random – twittermites don’t already exist as in-universe fiction of some sort, and Apple Bloom and Discord got the idea independently.
 
I mean, for instance, I could see twittermites being some sort of imaginary archetypal pest farmers might compare their troubles to – sort of like something worse than any actual vermin, so worst come to worst they can console themselves that at least it’s not twittermites they’re dealing with.  

 
Or some sort of in-universe myth or a cryptid or something like that.
 
The point I’m trying to make is that while I definitely think it’s a lot likelier that twittermites exist in-universe now, I can still see that arguments for them not actually existing might hold up – after all, we’ve only seen them in a dream and in one of Discord’s chaos extravaganzas, and Discord’s not exactly a stranger to making up stuff that’s absurd even by the standards of the show’s creatures and magic. There needs to be a way for both him and AB to have come up with the same concept, but I don’t think that that’s too hard to explain away if we assume that twittermites or something similar has some preexisting presence in Equestrian culture, even if no such creatures actually exist.

@Frustration in Excelsis  
Fair enough. (I could argue that all those improbabilities arguably make it more likely that Discord would do that, but then I’d just be arguing in poor faith. You know, as opposed to this parenthetical discourse, which is totally fine. :P )

@FanOfMostEverything  
Fair enough, but that would beg the question of why he’d be interested in reaching into a random filly’s subconscious to copy a single type of creature.
 
I don’t dispute that he could do it, but I’m not really seeing why he would want to or why he’d know or want to look there, specifically.

Possibly. Given Discord, he might have pulled them out of Apple Bloom’s subconscious.