The first 103-forms were bred from Compassion, (PROSE: The Shadows of Avalon), but eventually the help of Compassion was no longer required, with new 103-forms being created through the mating of older 103-engines with bull-engines. Each 103-form was individual, with its own personality and quirks caused by deliberate mutagenic factors in the breeding process. (PROSE: The Book of the War)
They could appear humanoid, (PROSE: Alien Bodies) and this was their default form, though with preparation they could transform themselves into the likeness of any human-sized being. They also carried weapons in their internal dimensions. However, none of the 103-forms were as powerful as the 102-form, suggesting that the cradle-engineers deliberately handicapped their creations.
Despite their sentience, each 103-form had a symbiotic relationship with a pilot from the Great Houses, albeit ones that acted more as a companion than a captain. These pilots lived inside the 103-form's internal spaces, and, while the ships anchored the pilots to the physical continuum, the pilots anchored the ships to the political continuum, giving them a context for social understanding. (PROSE: The Book of the War)