Gelatinous Pillion

The Winged Slime gives you a short flight, a glide once that runs out, and a hover on the down key, which between them cover most of what a low-tier pair of wings does. Since a mount costs no accessory slot, the practical result is that wings and boots both come off and those slots go to damage or defense. The trade is ground speed: on flat land it is a little slower than decent sprint accessories.

The mount and any wings you are wearing draw from the same meter. Burn one and the other is empty too. Dismount before it runs dry, though, and the remainder is banked, so alternating between the two effectively doubles your climb. Boost wings combine with it in a nastier way still: hold up under Jetpack power and swap to the mount in midair, and it launches upward far past its normal ceiling.

Landing on an enemy from above deals summon damage, so a Summoner Emblem or any other minion booster raises it. The mount falls very fast and does not widen your hitbox, which makes it the quickest way down a hellevator and a decent dodge in a boss fight. Top horizontal speed is higher while falling than while walking, so keep jumping if you are covering distance.