Thunderlane wanted to be happy that his little brother finally got himself a special somepony. However, Rumble's some "pony" wasn't really who he expected. For one thing, he was a dragon. Another thing was he was still roughly half Rumble's size.
I'm a shameless neckbeard who has a knack for writing a LOT of smut. I have an Official Patreon page (Patreon.com/TheVClaw), and I also take commissions to help support myself as a freelance writer.