Redstone Bug

Redstone bugs were a hostile mob present in Java Edition 2.0.

Spawn
Redstone bugs spawned close to redstone which was rapidly oscillating between strengths.

Behavior
Redstone bugs, effectively being reskinned silverfish, shared all of their behavior, including them entering blocks.

Entity Data
Redstone bugs used the ID of silverfish, different from them only by a newly-added Color NBT tag, which could have any color applied to it vis editing, not just the redstone shades.