So, I've been a fan of the Monster Hunter series for some time now, and especially the themes that play when you fight the monsters themselves. As I doubt you all would be interested in the specific anatomy of each and every monster in the series, I'm just going to post the links to the themes I personally like the most as well as a short description of the monster it is attached to.
This high-energy theme belongs to Zinogre, the Thunder Wolf. Zinogre is a very unpleasant monster to encounter because, despite it's impressive size, it is extremely agile. Seriously, Sparky here does backflips, mid-air rolls, and back slams, all while charging up with lightning. Once he is fully charged, enters a kind of super mode where all of his attacks are faster and have added lightning damage. One more thing. Whatever you do, do not enrage him while he is charged up. You will regret it.
Ah yes, Brachydios, the Crushing Wyvern. This black-scaled T-rex knockoff is one irritating monster. Not only does it jump about like it wants to be a kangaroo, but it's hands and the horn on it's head are coated in an explosive green slime. If you get that stuff on you, you have to waste a buch of time dodge rolling to get it off. The gear you can make from him is kinda cool though.
Gammoth, the Giant Beast. This furry pachyderm, according to MH's lore, was worshipped in the past as a mountain god. I can honestly see why. First of all, Gammoths are huge, at least three times the size of your average African elephant, and thats just the females. Secondly, Gammoth maintains a zero tolerance policy to any and all predators, which includes you. Third, it can snort giant snowballs at you from it's trunk. Need I say more?
Poor Astalos, how you try. Known as the Thunder Wyvern, Astalos is plagued with the problem of being weak for when you meet him. Despite an admittedly cool design, an awesome theme, a downright disturbing ecology video, and a penchant for using green lightning, he is simply a pushover by the time you meet him, especially compared to things like Gammoth and Glavenus. Speaking of whom.......
Glavenus, the Cutting Wyvern. A mutant Carnotaurus with a giant sword for a tail. It's theme is the worst of the MHGen Flagship monsters in my opinion, but the design for Glavenus is downright terrifying, and it can vomit magma at you.
Ah, Mizutsune, the Bubble Fox. This sleek, swift Leviathan has one of the best pieces of music attached to it out of any from the MH series. Seriously, it sounds like some sort of tribal wardance. Also, it attacks you with bubbles. I couldn't make that up even if I tried.
Shagaru Magala, the Heavenly Revolving Dragon. This arrogant prick of an Elder Dragon is one of the most challenging fights in MH4U, but it oh so satisfying to put the golden nightmare in it's place. This this is fast, agile, infects you with an extremely unpleasant virus, and is quite well aware of how powerful it is. However, if you can kill it, the equipment you can make from it is well worth it.
Ceadeus, the Ocean Dragon. The last fight in MH3's main story, and what a grand fight it is. You start off in an underwater cavern, swimming alongside Ceadeus(who is the size of a blue whale), trying to give him a shave, while he just casually swings his head from side to side out of annoyance. The first theme, Lunar Abyss, plays during this section, a perfect reflection of Ceadeus' calm, relaxed attitude towards you. If you break his beard before he escapes, he will enter the second part of the map and the real fight begins. The second stage involves combat in an underwater city against the pale dragon, with him attempting to kill you via tail slaps, biting, and a high-pressure water laser. The second theme, Moonquake, is playing in this part, quite well representing the fact that you have angered the giant of the sea. His gear is also something to behold.
Rajang, the Gold Lion. He isn't really a lion, more like an overly aggressive mandril with horns, but oh how this monster triggers people. Firstly, he is fast, agile, and hits like a truck. He also holds the (fan given) title of "MH's Son Goku", and with good reason. If you are unfortunate enough to enrage him, his hair stands up, turns gold, and he starts throwing whole hills at you and vomits a golden laser at you.
The first real monster you fight in MHGen, the Great Maccao, the Jumping Dog Wyvern. It's really not that strong, but it jumps about, a lot. it also stands on it's tail, using the opportunity to kick you and leap at you. It can be a pain, but as I said, it's weak, so it goes down quickly.
Ah, Mi Ru, the Black Fox Wyvern. Easily my favourite monster in the Frontier series, and with good reason. First, it's theme, much like Mizutsune's, has a very tribal/japanese theme to it. Second, it has a cool design. Third, it can actually change the size, shape, and density of it's body, allowing it to adapt to anything the hunters throw it's way. It's truely is a spectacular monster, and one I dearly wish they would add to the main series.