Monsters

Monsters are created by X.A.N.A. in Lyoko, to fight the Lyoko Warriors, protect X.A.N.A's activated towers, and to be used as X.A.N.A. sees fit. There are a wide range of monsters and most of them are unintelligent but it is shown that the monsters can be inteligent. They are directly materialized into Lyoko. According to codelyoko.fr, X.A.N.A. uses a base template which is uses to make it easier to create new monsters.

Virtualization

It is possible that they are created from a basic program, with all their characteristics, allowing X.A.N.A. to create many monsters, all almost identical. This would correspond to the program created by Jeremie to virtualize vehicles. In some episodes, such as Marabounta, X.A.N.A. virtualizes creatures directly in the field as Jeremie does with the Lyoko Warriors. it is shown that Monsters take up resources on which ever supercomputer they are located on, as seen with Kolossus since it can only be mobilised by X.A.N.A. with multiple Replikas as sources of energy. When monstera re mortally wounded or fall into the digital sea, they simply devirtualize. The destruction of their virtual envelope corresponds to their complete disappearance within the virtual world.

Monster POV

Most monsters have the ability to aim their laser, though they do not seem to do so often. This is shown from the monster's point of view, which is dominated by a trisected, white circle. When targeting something, an upside-down, red triangle follows what they intend to shoot. When it locks on, the red triangle centers within the circle and three red lines connect each vertex of the triangle to the circle. Only a few monsters have directly demonstrated this targeting system, but it is safe to assume that others utilize the same system.

Monsters on Lyoko

U - monsters bow — X.A.N.A's monsters bowing to the Lyoko Warriors, as seen in the episode Marabounta.

Kankrelats, are X.A.N.A.'s most common monsters, they usually attack in large groups. They are much faster than bloks, and are fairly hard to hit.
Hornets were the first monster that was able to fly, they are one of the few monsters to have multiple attacks. They can either fire a laser or shoot acid.
Bloks are one of X.A.N.A.'s most used monsters along with the kankrelats. bloks have three different attacks, standard laser, rings of flame, and freeze ray.
Krabs they are generally found in groups of 2 or 3, they use their legs to stab the warriors and shoots the warriors with it's eye. this is the only monster that can be destroyed by cutting off it's legs
Megatanks
Guardians this is the hardest monster to destroy since it has no weak points, Guardians are used to capture someone. the only way to destroy one is to use a clone to trick it.
Tarantulas were first introduced in New Order, they were depicted as XANA's deadliest monsters, they are excellent shot, and are very hard to hit.
Creepers
Scyphozoa
Mantas
Sharks
Kongre
Kalamar
Kolossus

The Mantas provide another part of the answer: when the heroes first access the interface to Carthage, Mantas appear as if they are 'hatching' from the walls of Sector 5 - they seem to spring from within the digital data. If they are created from a program, in any case, they are not virtualized (within Sector 5 at least).