Just as Rainbow Dash babies Astra, Clear Sky babies Thunderbird, because they both know how much he loves it. Thunder will deny it though. Well into adulthood he has a huge soft spot for his mom, and she's one of the only ponies he's not downright rude to.

Thunder inherits Sky's unicorn tail, a physical trait almost exclusively seen in unicorns. Even among the children of interspecies couples, it's pretty rare to see a non-unicorn with a lion tail.

Thunderbird is barely even a year old here, but he's noticeably much larger than Astra in the linked picture above, even though Astra is nearly 6 there. Thunder was the biggest foal, but he ends up the shortest out of all his siblings. (Astra happens to be the largest once she hits her teenage growth spurt)

Dash and Sky hold their kids by the scruff of their necks (only when they're young though). The skin there is super stretchy and it's more comfortable for the kiddos than it looks. It doesn't hurt at all!

Even though Thunder's wings develop super fast, he's not able to fly until he's about 2 or 3 (although he certainly tries). His internal pegasus magic has to catch up with his physical growth before he's actually able to fly (same for all pegasi).

He gets his spots and markings from Clear Sky's side of the family.