Companions are horse-like beings that are always white, have blue eyes, silvery hooves that sound like bells, and always run very fast and have much endurance. The first ones were Grove-born, and there were more of those than were born normally, but that is not the case several hundred years later during the Last Herald-Mage trilogy, and then later during Selenay's time. In those times, the only Grove-born Companion is the Monarch's Own Companion. He is always a stallion, and if he dies, then another will appear in the Grove and Choose the current Monarch's Own if he/she is still alive. This Companion never seems to age, and it is possible for him to have many different Heralds, because when his current one dies, he goes out to search for a new one. The Companions who are born the normal way live as long as their Heralds, and are reincarnations of previous Heralds. An example would be Herald-Mage Savil, Vanyel's aunt, who became Companion Sayvil, and Chose Kerowyn. The Companions did not want their Heralds to know this, so they blanked it out of their Heralds' minds until it was discovered in Selenay's time.
If their Chosen Herald has Mindspeech, and most do, then the Companion can speak mind-to-mind with the Herald. It was not infrequent for Heralds to get stares for replying to something the Companion said, or suddenly breaking into laughter, while the Companion whickered in it's own version of laughter.