Familiars

Most mid-level magic users will obtain a familiar at some point. Users will usually meet their familiar during their apprenticeship, although it is not necessary or expected. These familiars will grow with them as they transition into their next phase. Between the four types of magic users, Mages are more likely to have a familiar, Seers, the least. 
Some apprentices will set out on a journey to specifically find their familiar. There is no one specific designated familiar for a magic user, but they're not just chosen either. A connection through magic must be made. In some cases this requires time, in others it requires completion of a task by both parties, and still yet in other instances a connection is made on sight. Bonds between the magic user and their familiar vary in intensity depending on the person and the familiar. Some are deeply connected while others may exist but are not used. 
Familiars work with the magic user to enhance their power, provide comfort, and take on magic the user may require of them. For example, Healers are known to test their potions on their familiar. Due to the bond between the magic user and the familiar, they are deeply in tune with the needs and desires of their masters and will sometimes act on their own accord if they feel it is in their master's best interest. 
Familiars come in many different shapes and types. Some are more traditional: dogs, cats, mice, frogs; and some are less traditional: hamsters, dragons, spectres, plants. All that is required is a magical connection to be considered a familiar. Due to this attachment, familiars are blessed with long life - usually lasting the same lifespan as their masters - and a connection to magic as well. 
While a familiar provides benefits to a magic user, it is sometimes a burden as the death of a familiar can cause a deep scar in the magic user and restrict their access to magic. Sometimes a new familiar can be found and to make a connection with, but those cases are rare. As such, magic users take very good care of their familiars. Usually the benefits outweigh the risks.