I suddenly got an idea about how Stained Glass came to be and I just had to draw this scenario
 
I’m super proud of this and absolutely love how it came out ^^
 
Warning bad writing ahead
 
Rainbow and Dumbbell wanted to have another foal but they already had a really hard time with having Color Theory. They had tried for a really long time to have a foal but it never worked out, until one day when Rainbow discovered that she was finally pregnant. They didn’t want to go trough all of that again so they asked for Discord’s help and if he knew how they could do it. Discord helped them buuut it did not go the way it was supposed to, a fact Rainbow and Dumbbel only knew once Red was born and it was quite shocking at the time. They thought that they were expecting a normal happy little pegasus foal and not a Ursa Major cub. After the initial shock and a few words with Discord they decided whatever a child is a child and they will love them no matter what. Luckily Red wasn’t too different from pony foals.  
Color Theory didn’t treat Red differently than other ponies and Rainbow was pretty sure he thought that they were a pony. Red and Theo have a 4 year age difference and they get on really well. Theo is one of the few ponies that Red trusts and feels comfrotable with, excluding their parents.