The Hotaru is a firefly-type Badnik with a yellow and pink-colored head, small antennas and front legs. Their abdomens is mainly an electric light bulb while their thoraxes are thick and red.
Hotarus come in two types depending on their Rounds. In Stardust Speedway, Hotarus fly around for a short while before locking themselves into position above and below. They then shoot a line of electrified balls at the player before flying away, never to be seen again. The player can destroy them with a Spin Jump before this happens. In the Bad Future or sometimes the present timeframe of Stardust Speedway, these Badniks will be worn down from age, showing broken light bulbs. Their attacks can also not harm Sonic and are more akin to harmless flashes.
In Metallic Madness Zone 3 there are three Hotarus in a closed room immediately before the final boss. As they each start appearing, they fly in linear direction, shoot an electrical laser straight down to the ground, and then fly to a different part in the room to repeat the pattern. The player is required to defeat the three Hotarus in order to reach the boss. These Hotarus are always present in this room, even in the Good Future for Zone which would ordinarily remove the enemies. Like every Badnik in the game, Hotarus are powered by Little Planet flower seeds which will pop out of them when they are defeated.
- Hotarus in Metallic Madness Zone 3 are first notable enemies, that are required to be destroyed in order to pass through the room or proceed in the Stage. This game mechanic is utilized much later in games such as Sonic Heroes, Sonic Rush and Sonic the Hedgehog (2006), where some groups of enemies have to be defeated in order to open the door of the closed room or activate certain item separately or inside the Case.
- Hotaru have some similarities to the Flasher Badniks from Mystic Cave Zone in Sonic the Hedgehog 2, as sharing the same species being based of and both of them using the electricity.
- "Hotaru" is the Japanese word for "Firefly".