Some entities have very specific requirements...others give us freer reign. [sic] Frequently we form the Manifestation-Body to the mental image of the beholder. 
Manifestations are a race that gives M-Bodies to abstract beings who wish to manifest, as well as Galactus, Celestials, etc. M-Bodies are quasi-physical and change depending on the person looking, unless the entity specifically wants to look a certain way.[1]
History
M-Bodies of abstract beings, a Celestial, a Watcher, Galactus, and the Stranger
The Manifestations are a race of Living Fractals from the Dimension of Manifestations. They are capable of shaping themselves in an infinite manner, serving as Manifestation-Bodies (also called M-Bodies) to Abstract Beings, with whom they share a symbiotic bond as they provide power to feed the Manifestations. The newborn Manifestations are given Finite Beings, such as Galactus and the Celestials, to practice on before being allowed to manifest Abstract Beings. Said Finite Beings enlist their services when they cannot physically attend an event.
M-Bodies of abstract beings, a Celestial, a Watcher, an Elder, Kronos, and the Beyonder
The entities the Manifestations represent have input on how they like to be represented. Some have very specific requirements while others give the Manifestations free rein. Frequently, Manifestations form the M-Body as the mental image of the beholder (thus, the appearance changes depending on the person looking). For beings like Galactus or Celestials, these M-Bodies do not have to be a reflection of their physical size: Quasar noted that the M-Bodies of the 15 feet Stranger, 30 feet Galactus, and 2000 feet Celestials are all the same size.
Although they have no true leader, the Manifestations negotiate through a primary diplomat, or Prime Manifester, who is currently Anthropomorpho.[1]