Speculatores and Exploratores were the scouts and reconnaissance element of the Roman army.
The first permanent imperial bodyguards were the Corporis Custodes (also called Germanic or Batavian bodyguards). They were probably involved in the death of Emperor Nero, and therefore from that moment on amortized as bodyguards.
His successor, Galba, promoted his own bodyguards, the Speculatores, to imperial bodyguards. This situation continued until emperor Trajan promoted his own bodyguard, the Equites Singulares, to imperial bodyguards.