It is completely unknown, even by Unicorns, where they are native to. They are frequently found with Keepers who act as their hands, and work magic unicorns can't.
Unicorns tend to live in mountain forests but can be found in most environments. They are diurnal. Their primary diet is fruit but they will eat other parts of plants, being careful not to eat too much of any one plant that it is killed.
Gestation is roughly a year and weaning happens a year after birth. Children stay with their mother until they are 50 or so after which they may or may not stay with the herd, this is also when they start searching for a keeper to bond with if they wish. However they are not considered adults until they reach their first century, and will generally wait to choose a mate until then. Unicorns may or may not mate for life but generally if a pair separates neither will chose another mate. After their first 1000 years their color changes to all white and they gain the title of Silver and become the elders and leaders of their Splendor.
Unicorns are functionally immortal in that they do not die of old age but can be killed by accident, murder, or spending too much time in a contaminated area. While killing a unicorn will destroy the killer (some believe even the soul is destroyed), they may choose to will their bodies to be used for magical purposes after they die, usually to their Splender's Keepers or another person/group the unicorn was close to.
They are horse like beings who are roughly 5' (15 hands) at the shoulder with a single spiral horn on their forehead and cloven hooves. Coat/mane/tail/eyes of young unicorns can come in any color. After a Unicorn has reached their first millennia they turn solid silver color. Horns look like clear crystal when not being used and glow with a white light when the unicorn uses their magic.
Magic
They are the single most powerful magic wielders in the known worlds. Their magic lies in healing, purification, growth, and walking between the realms. They are also telepathic but they only use this with each other and keepers, mostly the one they are bonded to. Outside of what is listed, it is almost impossible for Unicorns to learn other types of magic.
Also, outside of purification, magic doesn't work in contaminated areas or on contaminated beings. Contamination can be caused by any number of things including: a battlefield or other area with a lot of violent death, places where death magic (unless the sacrifice was willing) has been performed, or areas that are highly polluted.
Most of them cannot shift when the sun is in the sky and for those who can it is difficult.
Memory
Before Keepers, due to the lack of hands, Unicorns memorized everything rather than writing it down. Now Keepers are able to write down things for them but most Unicorns still use their memory. Partly cultural but partly practical, since not all Unicorns work with Keepers and even those with Keepers may be separated for some reason. As such Unicorns in general are known for their great memories...to the point were a properly trained Unicorn has to choose to forget something they know longer wish to remember
A Splendor is a group of unicorns. Usually between 20-40 but can be smaller or (rarely) larger. Usually lead by one or more Silvers, if there are no Silvers then the oldest lead. Some Splendors are nomadic but others keep to a large enough territory to feed everyone. There is usually a community of Keepers that live with them.
Several nearby splendors group together during the solstices and equinoxes where they celebrate the changing of the seasons with games, stories, and magic. Sometimes young unicorns join other splendors during this time.
Unicorns are usually peaceful but any trespasser will quickly find themselves surrounded by the Guardians, unicorns and keepers who guard their territory or splendor, who can be quite aggressive in removing the trespassers. More nomadic splendors tend to be less defensive than those with set territories but going near the nursery without permission is never a good idea.
Unknown, even by Keepers. Everyone is fairly certian they do have something but they are very quiet on the topic.
No one knows if Unicorns are native to Haven, Earth, or some third place. Not even the Unicorns themselves. They, however, are the first to travel between Earth and Haven. They also are the ones who guided the other magical beings during the Migration. They (and their keepers) were the only group on both sides during the mage wars. With that said they didn't fight, instead they healed and purified the land after battle. They requested to be exempted from the Daughter's spell but where denied. This angered them and they are believed to be the reason the Daughters are cursed to take mates, although it is unknown if it was on purpose or by accident. Despite this, in current day, Unicorns get along well with the Daughters get along.