Over the period of captivity of the created chimera, a lot of experiments were conducted with its blood.
It has advantage on saves against being charmed, and magic can’t put it to sleep.
It is proficient in the Perception skill due to the blood of a predator.
It is able to digest food, however it does not serve a purpose for itself.
Trough an injection of a bat-blood like substance, the chimera has developed an echolocation system that allows it to be blindsighted up to 10 feet away.
In addition it is immune to poison due to the experiments made to it.
It also developed the ability of an nocturnal animal.
It can see in dim light within 30 feet of itself as if it were bright light, and in darkness as if it were dim light. It can’t discern color in darkness, only shades of gray.

Chimeras don’t need to sleep. Instead, they remain semiconscious, for 4 hours a day. (The Common word for such a state is “trance.”)

Its Dexterity score increases by 2.

It can speak, read, and write Common.
A chimera is not meant to learn languages. However, due to its adaptability it has learned to read and write the language of common folk.

In the case of a creature magically produced by alchemy and used as a test object for various experiments, it is difficult to define its age and expiry date.

A chimera does not serve the purpose of belonging to an alignment. Its only purpose ends in serving the desire of its creator.
But it also has free will, desires, wishes and urges.

a chimerical creature has about the size of a human with 5 to 6 feet. Its size is Medium.

Its base walking speed is 30 feet.

Created for the purpose of defending its master, it has the ability to climb walls. Because of his claws, it has a climbing speed of 20 feet. In addition, its claws are natural weapons, which it can use to make unarmed strikes. If it hits with them, it deals slashing damage equal to 1d6 + its Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.

As an action, it can change its appearance and its voice as one of a human it remembers. It determines the specifics of the changes, including its coloration, hair length, and sex. It can also adjust its height and weight, but not so much that its size changes.
It cannot make itself appear as a member of another race. It can't duplicate the appearance of a creature it has never seen, and it must adopt a form that has the same basic arrangement of limbs that it has. Its clothing and equipment aren't changed by this trait.
It stays in the new form until it uses an action to revert to its true form or until it dies.
However, since it is only an experimental form of the shapeshifting of a changeling, it can only use this ability once per long rest.

It was created with a very flexible tail, serving the purpose to attack and fend off attacks.
The tail is a natural weapon, which it can use to make unarmed strikes. If it hits with them, it deals slashing damage equal to 1d6 + its Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.
In addition, a saving throw of CON 12 or higher must be rolled to receive no damage from its poison. (1d4 poison damage, halved on sucess)

Reaction:
(If a creature it can see within 10 feet of itself hits it with an attack roll, it can use its reaction to swipe its tail and roll a d8, applying a bonus to its AC equal to the number rolled, potentially causing the attack to miss it. This can only be used twice per long rest.)

The tail cannot be used to lift or use objects due to its shape and form.

The tail may support the full body weight of the chimera for up to six seconds 1 combat round) after which it must regain its footing or fall. (Essentially, it may hang using its tail for one round, at the start of the next characters turn it falls of it remain hanging.)

It may assist in climbing if applicable to the situation (such as griping tree limbs, lengths of metal, or anything it can wrap around) giving the chimera proficiency in any ability check made related to climbing.

Experiment 137 MDMOA discarded. It shows deficiency in its regenerative capabilities.

It does not regain Hit Dice over the course of a long rest. Instead, it must consume flesh and appendages from its fallen foes. Each feast being worth 1d4 of healing.

Due to a genetic defect, experiment 137 is not able to keep itself over water really well.

it also seems to have developed a weakness against ice, and therefore fears it.

It has absolutely no idea how to handle money or valuables.

It does seem to be quite dysfunctional to retain any historical memories in its mind.