As the name replies, Egg Ball is a giant Egg Mobile with another dummy ball. It's mostly blue with some gray and yellow. The dummy ball is mostly gray with some red and orange.
It's a tricky boss fight as Dr. Eggman in his mobile and a "dummy ball" go through tubes and the player must hit Eggman; hitting the dummy ball will result in losing rings (unless they have a barrier which in that case they'll lose the barrier instead). The more hits Eggman takes, the faster Eggman and the dummy ball moves through the tubes increasing the difficulty especially for players that do not have quick reaction times. To overcome this though, it is recommended that the player pauses the game just as Eggman and the dummy ball exits out of the pipes.