[I’ve extended “monsters” to “various mythical creatures”, I hope you don’t mind.]
Asahi: Asahi would be a Deva. Generally speaking a Deva is a benevolent and supernatural being that bears a lot of power within itself. This would fit Asahi very well as he’s very keen on being in a relaxed and calm environment. He has also been shown in both the manga and the anime as somebody who helps people without having to be asked. At the same time he is a very powerful and most definitely threatening opponent in volleyball, which shows that within his peace-loving soul there is a strength that shouldn’t be underestimated.
Sawamura: Sawamura could be a Centaur. Despite the fact that in most descriptions centaurs were depicted as ruthless brutes, there are also stories talking about their ability of being excellent and talented teachers. Sawamura would fit really well in this category because on one hand he does seem a little raw around the edges and at the same time he has also presented himself as a excellent leader and helper to his teammates.
Sugawara: Sawamura could most likely be a Caipora. Caiporas are describes as mischievous creatures that help those who treat their environment/nature well. Sugawara is someone who appears to be incredibly friendly on the outside, but he definitely has another side to him that could be described as slightly devilish. He is quite easygoing and fun to be with, which would appeal to a lot of descriptions of caiporas.
Tanaka: Tanaka could be an Egyptian type of god, called Bes. Bes are described as small lion-like dwarfs that live in the houses of humans, protecting the children during childbirth. They appear to be very cheerful, loving to make small children laugh and interacting with the other humans. It is also said that they’d play harmless pranks when upset. This could fit Tanaka, who’d very likely be someone who loves to play and interact with children. In general he’s a fun person to be with, who also wouldn’t mind to play a trick on someone from time to time.
Nishinoya: Nishinoya could be a Thunderbird. Thunderbirds were said to have incredible strength, being able to create thunder and stormy weather with their wings. It is also said that they protected humans they trusted whenever they were in trouble. This would fit Nishinoya’s personality, since he has a very energetic personality and is very keen on helping and guarding his team during matches. Despite his height he shouldn’t be underestimated, as he definitely has the build and physical strength to be an intimidating opponent.
Hinata: Hinata could be a Quilin. Quilins are symbols of good luck, protection and good omens and Hinata’s persona suits this perfectly. He is a huge enrichment to his team and in one way or the other he probably has brightened everyone’s day at least once by being the fidgety, bouncy and happy person. He gives his teammates hope and motivates him by bouncing right back up once he has fallen.
Kageyama: Kageyama could be represented by Horus. Horus was an Egyptian god who controlled the skies and presented the souls of the dead to Osiris, the ruler of the netherland. This description depicts Horus as a very serious and dignified character, which would definitely suit Kageyama’s personality as well. The fact that Horus was the ruler of the skies could be taken as a parallel to Kageyama being able to control the flow of the game in his position as a setter.
Yamaguchi:Yamaguchi has the potential to be a Familiar. Familiars were said to be protectors and helpers of witches and magicians, being able to take on various physical forms. Even though Yamaguchi doesn’t appear to be somebody who could achieve a whole lot in his current state, he definitely does have the potential to become somebody worthy of being feared. Familiars also seemed to be quite insignificant until it came to protecting their master. With the right spell and person they could receive the strength they needed to be victorious. The same goes for Yamaguchi. The more support he is going to get from his team and friends the more likely he is going to be able to perfect his serves.
Tsukishima: Tsukishima could be connected to the Egyptian god Aapep, who symbolized death, the cold night, chaos and storms. Aapep was a vicious creature that was feared by many. This being could represent Tsukishima’s emotional and mental state from the time he found out about his brother to him slowly becoming Hinata’s rival. Tsukishima is somebody who has the tendency to brood, he is quite sarcastic and ruthless, fitting the description of this vicious snake very well.
Oikawa: Oikawa would be a Hippogriff. The Hippogriff is said to be the son of the griffin and connected to taking part in numerous heroic actions throughout mythology. At the same time a hippogriff is a sympol for making the impossible possible, which would fit Oikawa’s attitude perfectly. He, as somebody who wasn’t born a “genius” and had to work hard for his success in volleyball, has made it incredibly far and as we have seen in the manga, his most worrisome opponent is Ushijima, the griffin.
Kuroo: Kuroo could be connected to the Androsphinx. As a symbol of power, knowledge and mysteries the androsphinx was a very witty being, often questioning humans with riddles. Kuroo could fit this very well, as he is somewhat of a prankster himself, often saying things that have more than one meaning to them. He has also been portrayed showing a creepy, mischievous smile, and most importantly, the imagine of a feline would immediately pop up in one’s head when thinking about Kuroo.
Bokuto: Bokuto would be a Snow Lion. A snow lion is said to symbolize fearlessness and unconditional cheerfulness wich would fit in perfectly with Bokuto’s personality. He’d most definitely a risk taker, preferring to try it at least once before giving up on it (even though he wouldn’t give up). At the same time he seems to be in a very good mood most of the time, which would spreach out to his teammembers and sometimes even to random strangers.
Ushijima: Ushijima would be a Griffin. A griffin is describes as king of all creatures, which would definitely suit Ushijima’s current status. He is very disciplined and at the same time he knows that he is superior to most other players. He trains more than anyone else in his team, presenting himself as the ultimate leader. It has also be shown that he seems to be one of Oikawa’s biggest rivals.
Kenma: Kenma might be a Lynx. It’s described as an elusive creature, the “keeper of secrets” with the ability to see the hidden truths in things. This would fit Kenma very well, as he prefers to stay in the shades, unnoticed by anyone. He is also a very observant person, who probably even sees more than he’d truly like to. During games he shows this ability even more, already starting to plan a strategic counterattack. I feel like he’d also be able to find solutions to problems that would emotionally inhibit other people. At the same time he could be very blunt about telling others of their flaws.
Aone: Aone could be a Dactyl. Dactyls are described as creatures that excelled in magic and ancient smithery, often teaching mathematics, the alphabet and metalworking to humans. Aone seems to be somebody who would prefer physical work that also had a connection to logic and statistics. He has also shown various times that he wouldn’t mind interacting with others and if it wasn’t for their fear of his exterior, this could actually happen quite frequently.
These are perfect!