Mega Latios is a Dragon and Psychic type Mega Legendary Raid boss. Optimal raid counters will be Ice, Bug, Dark, Ghost, Fairy and Dragon.
• Mega Latias is pretty cool and here is Mega Latios Raid Guide
• In order to get Mega Latios you will need 300 Mega Energy for the initial evolution. Once you Mega Evolve a Pokémon for the first time, Mega Energy will no longer be required for that specific Pokémon to Mega Evolve again. Instead, the Pokémon will become exhausted after its Mega Evolution and need to rest before being Mega Evolved again. In other words, there is now a cooldown period after you Mega Evolve a Pokémon. You can level up your Mega Pokémon in order to decrease this cooldown period.