Egg Serpentleaf is the boss of Sylvania Castle Zone in Sonic the Hedgehog 4: Episode II, piloted by Doctor Eggman. It looks like a snake combined with a flower. It can transform into a flower and it has a gun below the Eggmobile. Before the boss battle starts, Sonic and Tails find Dr. Eggman riding in his Eggmobile, but he instantly flees. When they catch up to him again, Eggman plays a little trick on the heroes before revealing the boss itself.
When the battle first begins, the tentacles will occasionally swing across the ground to try to hurt Sonic and Tails. When it takes four (console) or three (mobile) hits, it will raise up two more tentacles, and Dr. Eggman will start shielding himself with the flower petals more. Otherwise, it is the same as earlier.
Finally, when he takes three (console) or two (mobile) more hits, Dr. Eggman will shield himself with the flower, but this time, the tentacles wrap around the stem, inserting some type of liquid into it, and the bottom of the Eggmobile will reveal a cannon. If Dr. Eggman is left undamaged for a long enough time, the weapon from the bottom of the Eggmobile with shoot out fire that will cover the entire ground. But with two (console) or one (mobile) more hits, the boss is defeated, and Sonic and Tails will hit the Egg Capsule, ending the Boss Act.
- There is a reference to Sonic 2 within this boss, as before the battle starts, Dr. Eggman raises the stone pillars from his boss battle in Aquatic Ruin Zone, tricking the player into thinking they're going to fight a repeat of said boss battle, only for the Egg Serpentleaf to reveal itself and destroy the pillars.
- The music that plays for this battle is the same as the background music for the boss rush in the E.G.G. Station Zone.
- This is the only Eggman mech boss in Sonic the Hedgehog 4: Episode II that doesn't require the Rolling Combo to beat.