I've heard it's an event in Pokemon Ranger, but, is it one of those permanently available events like Magearna? Also, which Pokemon Ranger is it on? I've heard it's just on the original, is that true?(Reason I'm asking this is because my brother has an old copy of one, and I'm probably gonna start playing it if he lets me.)
Yes it is. In the original, all you have to do is beat the game, go to ranger net, press r, x and left on the d pad, and then type in this code for the usa: P8M2-9D6F-43H7. Then this text will pop up: Prof. Hastings obtained a Manaphy Egg, but it was stolen by the Go-Rock Squad! Before they can do any harm with the precious Egg, recover it from the criminals! It was on the other ranger games, but it was a limited time event, and it has finished. However, on the original, you can still redeem it because it is a permanent event.
Source: https://bulbapedia.bulbagarden.net/wiki/Ranger_Net#:~:text=Pokémon Ranger,-Bulbanews has an&text=In the first Ranger game,unlocked after finishing the game.&text=A Manaphy Egg can be,after finishing the specific missions.
I hope I've helped!