The Seneschai are vaguely humanoid in appearance, with two arms, two legs and a defined head. Their faces have been likened to those of a human fetus, with small black eye-spots. Their waists and limbs are extremely thin, and their appendages appear to lack joints. They are a uniform light green in color.
When human settlers came to Hebron, they clashed with the empathic Seneschai; according to the Consul, the Seneschai were negatively affected by the settlers' thoughts of fear and greed, as well as by their inability to comprehend the minds of humans. As a result of this, and perhaps more direct acts of aggression, the Seneschai were believed to have been wiped out on Hebron.
SPOILER ALERT! The section(s) below are MAJOR SPOILERS.
Unbeknownst to the rest of the galaxy, however, at least some of the Seneschai escaped extinction. Many Seneschai Aluit were present on at least one Startree Biosphere constructed by the Ousters and Templars.