Following this video of Ghost Kick. Is there a way to defend and anticipate this kick? Is it applicable to real street fight? Is it the same as close range roundhouse, Brazilian kick and kick from the pocket?
The ghost kick as demonstrated here is actually taught in standard Taekwondo. I recognized it right away. You occasionally see these sorts of unusual techniques from traditional martial arts creep their way into MMA fights. It's good to see.
In Taekwondo, this sort of kick is done routinely as a way of not telegraphing the kick and decoying. You lift up your leg at your knee only, keeping your knee bent and pointing it towards your opponent's solar plexus. This causes your opponent to think you're about to do a front kick to his solar plexus. So his guard comes down from his head to his torso. Then, you turn your hip over and switch it up to a high round kick to the head. Since his guard is down low, his head is unguarded, and you score.
This way of chambering the kick in a standard front-kick chamber position before doing the real kick you want is all part of standard Taekwondo. You don't just have the ability to do a round kick here. You can do a hook kick on the other side of the head. One of my favorites was to switch it up to an "axis" kick which is kind of like a front kick, but twisted at an angle, and it would be to the opposite side of the head as the round kick would do. You can do an axe kick. Or a crescent kick. Or a side kick. There are many possibilities. And by keeping your chambered leg low and bent, you can recover more quickly if your opponent catches you by surprise with something.
The only problem here is that these kicks don't involve a lot of momentum and body weight behind them. They're flicky kicks. But with training, you can get your kick power up enough that it can do real damage. Targeting the jaw (the "button" in boxing parlance) will increase the odds that you can get a knock-out from something like this. Although, honestly, it's still unlikely you'll do much except confuse or annoy your opponent. But maybe that's fine if it allows you to follow up real quickly with something that has more force behind it.
I personally would never use this kick in a real fight. It's good if you have a long game like an MMA fight would allow for. Then you might use it strategically and mostly to create an opening for a stronger technique. But in real fights, the fight is over in seconds. If you use this kind of technique, you can easily become unbalanced and fall. All he has to do is grab a hold of you and move towards you while your leg is still out there.
As for how to defend against this sort of kick, it's hard to defend against it specifically because it looks like a front kick to the solar plexus. It's a decoy, and it prevents telegraphing where the real kick will land. You learn to deal with this in Taekwondo sparring after a while of getting tagged with it. It will force you to increase your reaction speed, for example. You'll need to anticipate that the kick is a decoy and keep one hand up high to guard the head while the other hand drops to protect the abdomen. Most of the time, a step to the side (either side) would work, because it causes your opponent to have to recalculate your position. Counter-attacking the moment you see the leg going up while using half-beat timing is going to prevent or intercept the kick before it reaches the chamber position. Stepping into your opponent just as he's beginning to kick is a great idea, because you can knock him over. Most people don't like that idea, because their groin is vulnerable to a potential front kick in that case. So to prevent your groin from taking a hit, you would pivot and shoulder-check as you're stepping in, but remember to keep one hand high to guard in case he slips that ghost kick through. And if he does slip the ghost kick through, be ready to grab it and perform a leg sweep. This should give you plenty of ideas for how to drill these defenses.
Just one more note about this kick. What I said is correct. You learn this as a front kick style chamber and then go from there. But in practice, you rarely see much of the front kick chamber position. In the video, you see that the ghost kick here smoothly transitions right to the final kick. You can't even see the chamber position as a distinct, separate position very well in that video. Maybe only a couple times do I see it more clearly in that video, for example at 0:03. And that's what you expect from someone who has trained this kick to a high degree. But when training to do this for the first time, you're going to break it up into two separate movements. Later on you'll smooth it out.
Hope that helps.