All names are taken from the American game manual, unless otherwise stated with the addition of (JP) after the name, indicating a Japanese phoenetic translation.
Beetle robots that simply roll back and forth across the ground and pose no immediate dangers to Sonic. This enemy only appears in the Green Hill Zone.
Piranha robots that jump from underneath bridges. Caution when approaching a bridge is advised, as well as possibly jumping while crossing it. They are only found in the Green Hill Zone.
Crab-like Badniks that first appear in Green Hill Zone. They shoot either bombs or some sort of laser balls from their claws. They also appear in Spring Yard Zone.
Robots that fly across from the right or left side of the screen, occasionally pausing to fire a single, large flashing projectile downward. Can be ignored and avoided most of the time, with the safest place to attack them from being above. They are found in the Green Hill, Marble, Spring Yard, Labyrinth, and Star Light zones.
These chameleon robots appear on the side of cliffs but are invisible until approached. Blue Newtrons transform into missiles and fire themselves along the ground, while green Newtrons make themselves visible just long enough to fire a single, large flashing projectile at Sonic. They appear in the Green Hill zone only.
Caterpillar bots that crawl back and forth along the ground. Their only weak point is the head, which can be jumped on or rolled into. Hitting them anywhere else will not only result in losing all your rings, but their four segments will scatter and bounce around the area and risk causing more damage. They appear in the Marble and Scrap Brain zones.
Spherical robots that typically appear from the left side of the screen down long declines. They cannot be destroyed in their spherical form and roll faster than Sonic can run, and they also hold the ability to jump in this form, so fast reactions are needed. At random intervals, they stop and briefly open up - This is the only chance to attack. They can be found in the Spring Yard zone only.
Piranha robots that move about underwater, and dive at Sonic directly if he approaches. They can be found in the Labyrinth zone and the third act of the Scrap Brain zone.
Bomb-shaped robots that walk back and forth along the ground or ceiling. As soon as Sonic approaches, they stop moving and start flashing; after a few seconds, they explode into a small shower of flashing projectiles - 2 in either direction. They can't be damaged, only avoided. They appear in the Star Light and Scrap Brain zones.
Mole robots that leap from the ground when Sonic approaches, and then simply move back and forth along the ground. Their drill bit noses are slightly visible above the ground, giving them away. They appear in the Labyrinth and Scrap Brain zones.
Pig robots that hang around at the top of inclines. They just hop in place and occasionally pause to fire a projectile that slightly bounce down the incline towards Sonic. They only appear in the Scrap Brain zone.
Bat Brain (aka) Basaran (JP)
Bat shaped robots that hang out on the ceilings until Sonic approaches, at which point they drop down and fly horizontally in either direction for a short distance, before returning to their roost. They only appear in the Marble Zone.
Spiked Hermit (aka) Yadrin (JP)
Crab style robots, similar in style to the Motobug. They are more deadly due to the fact they have spikes on top, meaning the only way to destroy them is to roll into them. They only appear in the Spring Yard zone.
Orbinaut (aka) Unidus (JP)
Sphere robots that float back and forth very slowly. They're orbited by four spiked balls, making them virtually impossible to hit until after you approach them. Once Sonic comes in range, Unidus fires off the spiked balls in rapid succession in a dangerous assault. Once it's complete, Unidus is open to attack. They appear in the Labyrinth and Scrap Brain zones.
These guys, color aside, look exactly like Orbinaut. The difference is that they will never fire off their spiked balls, meaning the best course of action is simply avoiding them. They only appear in the Star Light zone.