The Spartan bodyguard unit that supplied Leonidas with his chosen 300 men was the Hippeis.
The Hippeis—literally “horsemen,” despite fighting as infantry—was Sparta’s elite royal guard. It consisted of 300 young Spartan citizens selected for military service and assigned to protect the kings. In 480 BC, King Leonidas took this elite force north as the advance contingent sent to block Xerxes’ Persian invasion at Thermopylae.
The famous 300 were not the entire Greek force. Leonidas initially commanded several thousand allied troops from other Greek city-states, including Thespians, Thebans, Phocians, and Locrians. Herodotus also says Leonidas selected Spartans who already had living sons, helping ensure that their family lines would continue if they died.
A common mix-up is to imagine the Hippeis as cavalry because of the name. Spartan cavalry was limited and developed separately; the Hippeis served as heavily armed hoplite infantry around the king. Their final stand became one of the defining episodes of the Greco-Persian Wars.