The Gate Children

The Gate Children are entities that make up the collective being that exists inside the Gate in the 2003 anime series storyline. Shadowy and childlike, these primordial beings appear to have a strong connection to the Gate's function, serving as the owners of the umbral, prehensile limbs that emerge to deconstruct things and people that seek to pass through the Gate of Alchemy itself. Living between the two gates of the parallel worlds, Earth and Amestris, it is highly possible that they are humans who have died in infancy or before birth, based on the fact that one of these Gate Children was recorded to have escaped the Gate and reached Amestris as the Homunculus Wrath, relating upon his rebirth memories of his birth-mother Izumi Curtis.

As depicted in Fullmetal Alchemist the Movie: Conqueror of Shamballa, humans that pass into the Gate through artificial means become exposed to the Gate Children for an unnatural length of time and begin to meld with them. This mutation grants the host human body immense alchemical power, likely due to the Gate Children's existence as part of the Gate itself. Much like when using the Philosopher's Stone, the host can use alchemy even if they have no knowledge of it, but the transformation also leads to insanity - as shown in the case of Dietlinde Eckhart.

Trivia

 * Despite being created specifically for the 2003 anime, the Gate Children are invented to explain the shadowy, prehensile arms that appear in the manga storyline when the Gate is opened and when Father and Pride reveal their true forms. Additionally, it is likely that these beings are based somewhat off of the entity Truth from the manga storyline.