How about the sirens?

@Background Pony #8F21  
I know, but those are the visible references to take from all Grogar’s monsters.

I’m pretty sure Grogar creates many more kinds of monsters, than we can see here.

It seemed to me that Discord was directly a creation of Grogar.
 
As for the relationship, I understood that Baast is her first love, but she is DCD.  
After his death he took refuge in his dimension or, one day he met Cosmos who was his second love but she was of a toxic behavior which led him to distance himself from it.  
Breaking these two relationships we discover him as an antagonist but fluttershy succeeds in bringing him to his senses.  
They become a couple even if the show makes us feel comfortable on a question.
 
For my part I have asked myself the question for Celestia that they have these relations with Discord.  
For it is clear that they have known each other for a long time and that there is a certain alchemy between them.
 
 
I made myself a fan fiction to put all the pieces back together.  
Fluttershy being mortal, she decided to anticipate her departure by bringing Celestia and Discord closer together. Discord was not ready for this end, on his side he threw himself into a form to give immortality to Fluttershy.  
His solution was to steal Grogar’s bell, to absorb much of Cosmos’ power in exchange for his release.  
But it is at this moment that fluttershy and Celestia find him and call him to reason.  
After several discussions Cosmos admitted that it was an opportunity for her to redeem herself, that she had given these powers because they were the source of their rupture.  
From are side fluttershy issued a condition to this immortality by leaving a chance to Cosmmos.  
As well as given a party of this power to bring back Baast and give them more time together.
 
Well it’s a summary and I’m not really proud of the multi couple, but it’s Discord it can well be divided into 4 to satisfy these ladies.
 

 
VF:  
Il me semblais que Discord étais directement une création de Grogar.
 
Pour ce qui est de c’est relation j’en ai compris que Baast est son premier amour, mais elle est DCD.  
Après sa mort il se réfugia dans sa dimension ou, un jour il fit la rencontre de Cosmos qui fut son deuxième coup de foudre mais elle étais d’un comportement toxique se qui l’amena à s’en éloigné.  
Briser de ces 2 relations nous le découvrons comme un antagoniste mais fluttershy réussit à le ramené à la raison.  
Ils deviennent un couple même si la série nous l’aise sur une interrogation.
 
Pour ma part je me suis posé la question pour Celestia qu’elles son ces relations avec Discord.  
car on voie clairement qu’ils se connaissent depuis longtemps et qu’il y a une certaine alchimie entre eux.
 
 
Je me suis fait une fan fiction pour recollé tous les morceaux.  
Fluttershy étant mortel, elle décida d’anticipé son départ en rapprochant Celestia et Discord se qui marchait bien. Discord lui n’était pas prés à cette fin, de son coté il se lança dans une quette pour donner l’immortalité à Fluttershy.  
Sa solution fut de voler la cloche de Grogar, d’absorbé une grande partie des pouvoirs de Cosmos en échange de sa libération.  
Mais c’est à ce moment que fluttershy et Celestia le retrouve et le rappel à la raison.  
Après plusieurs discutions Cosmos avoua que c’était pour elle l’occasion de se racheté, qu’elle avais donné ces pouvoirs car ils étaient la source de leur rupture.  
De sont coté fluttershy émis une condition à cette immortalité en laissant une chance à Cosmmos.  
Ainsi que donné une parti de cette puissance pour ramener Baast et de leur donné plus de temps ensemble.
 
Bon c’est un résumé et je ne suis pas vraiment fière du multi couple, mais c’est Discord il peut bien se divisé en 4 pour satisfaire ces demoiselles.

G5 is the only thing that may reveal if Discord had any kids

I know the comics are a thing and all, but I prefer to regard them as a separate continuity and just assume Discord is some bizarre chaos magic spawned from the dimension he calls home.