The Thargoids are an insectoid race, more advanced than humans, and were present in the original Elite games. They seem to have vanished off the face of the galaxy in Frontier, however. Thargoids hid in 'witch space'-- the dimension you pass through when you travel through hyperspace. Their large, flat octagonal ships were almost indestructible, and although you could drop into witch space by choice, only a hero or a fool would actually choose to. Thargoids knew this, and would 'suck' crafts out of hyperspace and then attack them, stealing their cargo and killing them.
As you can imagine, war broke out between humans and the technologically superior Thargoids. In Frontier they are no longer around... why is this? Common theory is that they are just too exhausted with fighting the war with mankind and have retreated back to their part of the galaxy. However, traces of them remain.
The Thargoid ship that can be sighted in Frontier has come to be known as the 'Mirage'. It isn't a mirage, but then again it isn't an operational Thargoid ship either. During the Thargoid wars many ships crashed and were captured. These now reside at secret military installations across the galaxy, and you will be able to see them when you perform photo and bombing missions for the Federation or the Empire.
You cannot fly a Thargoid ship in Frontier, despite popular belief that you can, and nor are there any active Thargoids in the game. There is a hacked save game available, however, that features a Mirage ship that is named 'Starblade'. It can be downloaded from the Saves page. The Starblade is merely a hack, and not a real Thargoid ship. It also comes fitted with a Fighter Launch Device and has a mammoth jump range. The ship is so strong that it cannot even crash; its shields merely 'bounce' it off the ground! (Thanks to Steve Critchlow for the hacked save game.)
There exists a myth that you can actually fly a Thargoid ship in Frontier (I don't mean hacked versions), and this myth was not helped by David Braben himself claiming that there was a Thargoid ship to be found somewhere in the galaxy, and that it could indeed be flown. Many believe it was a trick to keep people playing Frontier, and maybe it was, but the ship does not exist, or in any case has never been found. Below is the procedure believed to enable you to fly the legendary ship. It's a complete fabrication. The statement "Travel to the edge of the galaxy, and before too long, you'll find it..." is laughably farcical.
There is one more ship to be found and flown in Frontier, and not surprisingly it's the best possible ship you could lay your hands on. Rumours have abounded since the game appeared that there was a Thargoid ship hidden somewhere in the game. Well, I can happily announce that this ship does in fact exist. It's called the Mirage and can be found at the edge of the galaxy. However, before you can find it, there are a few things you need to do. Firstly, you need to be 'Elite' status, so don't hope to stumble upon it early in the game. Travel to the edge of the galaxy, and before too long, you'll find it. When you know where it is, you only need three more things. The first is the top rating in the Imperial Navy, that of Prince. When you have it, the Empire's flagship will approach you and you'll receive a message that reads "The Big Man requires a mercenary for a right hand man. That's you". You have now climbed the ladder as far as possible. You'll also need a user ident from the Sirius Corporation. The only way to get this is from the Managing Director of the Sirius Corporation, who you should already know is a bit of a tough cookie. Finally you need a large amount of Military Fuel, so prepare to dump all your cargo and non-essential items. Then the Mirage is yours!
Notice how the two ships lie dormant on a platform. The military of both powers are most likely busy dissecting Thargoid craft to try and discover advanced secrets.
Thanks to Steve Jaqvaar for the four Thargoid pictures.