These monsters do not appear in the games proper, but still exist within the Monster Hunter universe.
Has no known picture but is seen as a item in games. Holding a Khezu Whelp drains a hunters health. Their only known use for hunters are to sell them for money or pickling them for food.
Young Rathalos / Rathian
Barioth are born from live birth, and are raised by their mother in dens protected from the cold of the Tundra.
After they are born, young Duramboros stay with their mothers, which form herds for their protection.
Young Zinogre stay in packs of Zinogre. They have more fur bodies compared to adults, which serves to protect their juvenile shells and expedite their storage of electricity. This fur sheds as the juvenile matures into adulthood.
Not seen in game. Young Brachydios will live near their parents and their territory, yet the adults will not show any parental care. The hatchlings will also pick up slime mold left behind by adults but this can be dangerous as their hides are underdeveloped and can possibly be seriously injured or even killed by the explosions of the slime mold.
Kokomoa / ココモア
Known as Kokomoa through item names and info. It is the young form of Gogomoa. Just like HC Gogomoa, HC Kokomoa have yellow fur and eyes. It is impossible to kill or capture Kokomoa as defeating it or its parent makes it simply dig underground. Upon defeat of Kokomoa the parent Gogomoa will go into a red aura rage where its attack strength and speed are greatly increased but its body become extremely weak to damage. It is possible to acquire the very valuable fresh Kokomoa fur from carving the parent Gogomoa after its defeat.
Unnamed Insect Monster
It is dubbed by players as either Poison Flashbug or Poison Thunderbug. It first appeared in Monster Hunter Frontier Forward.2. Upon defeat much like Great Thunderbugs it will drop a shiny and what you can get from their shinys are Flashbugs, Thunderbugs and Insect Husks. At HR100 and G-rank they share the same Neopteron items other Neopterons have such as Neopteron Stingers LV1-3 in quality, Neopteron Juice LV1-3 in quality and Monster oil. At G-rank drops are Neopteron Poison Stingers, Neopteron Blade Shell and Neopteron Pheromones.
Extinct And Unconfirmed Status
Igurueibisu / イグルエイビス
Originally a concept monster, Igurueibisu is now considered the first Bird Wyvern and is the ancestor of all Bird Wyverns. It was a predatory Bird Wyvern that hunted with its strong beak and talons. It is confirmed extinct.
Keputosu / ケプトス
Originally a concept monster, Keputosu is now known as the ancestor of all raptorial Bird Wyverns such as Velociprey and Jaggi. Keputosu was able to fly like some Bird Wyverns but it wasn't a very good runner like the raptorial Bird Wyverns. It is confirmed to be extinct.
Borudoru / ボルドル
Bajirisu / バジリス
Originally only a concept monster, Bajirisu is a relative of the Gypceros that instead of having rubbery skin had a hard shell on its back and tail. Unlike Gypceros, it couldn't use its crest to produce a flash. It was to Gypceros what Yian Garuga is to Yian Kut-ku, but is currently extinct.
Wyvern Rex / ワイバーンレックス
Originally a concept for Tigrex, It is now known as the ancestor of all Flying Wyverns though this possibly excludes Akantor, Ukanlos and Odibatorasu though they are believed to share the same ancestor if not originated from Wyvern Rex. Wyvern Rex wasn't capable of flying like Tigrex due to its small wings but Tigrex and it have similar spines. It is confirmed extinct.
Sherureusu / シェルレウス
Originally a concept only. Sherureusu is the extinct ancestor of Rathalos and is a further evolved Wyvern Rex that split from Tigrex and is currently extinct. Interestingly, Sherureusu walked in a similar to fashion to Kushala Daora and was also an apex predator.
Canopus / カノプス
Originally only a concept, it is now known as a extinct relative of the Rathalos and Rathian. It had a strong heavy shelled body and is thought to have been able to walk on all fours like Tigrex. The heavy shell was actually all just one piece and this armor actually reduced its movability.
Ancient Serpent / エンシェントサーペント / Enshentosāpento
Originally only a concept, it is a very unique Piscine Wyvern that surprisingly is thought to still be alive but has made no appearances in games. It is requested by the Guild to kill these monsters on sight from them being able to do large amounts of damage like Elder Dragons. As a Piscine Wyvern it is related to Plesioth, Delex and Cephalos.
Originally only a concept monster, it is the serpentine-like Piscine Wyvern that is the ancestor of Lagiacrus. It is said to turn to stone as it gets older and older while alive and, theorized to have attacked a few ancient civilizations. They normally glow from their entire body but from petrification and parasitic shellfish covering their body, they now only glow from their stomach. Its unknown if its extinct or still alive.