So I did a little research about them, Most of them says that a familiar is a Witch's spirit/animal guide/servant some says they're demons. They have so many different definitions but they share the same concept, it is something that guides a witch or someone. All I have found are just dots, and I want to connect them... I really don't believe that they are demons because we all know that people like to call anything that is strange and not the usual thing is a demon or a work of a demon, seriously. So If there's anyone out there who have knowledge about familiars and they really are and better yet that they actually have they're own familiar and that they can or have been proven or claimed that it is really a familiar.
If you search in ars goetia ,It's written that many demons can provide you with familiars,they'll help a witch in collecting all kinds of herbs,help her collecting all kind of items needed in spells,rituals etc,they too have knowledge in stones,gems to bring you luck etc,same goes for magician as well,as you too know some items are hard to collect but with them it could be done in an instant,so that's why many needs familiars,but some are said to be quite deceiving as well but it depends on your agreement with the demon.
ooh.. it'll be Gold if you have one, Imagine all the knowledge you can gain . . I wish someone who has an actual familiar would tell me more about it i dont know why this kinds of information is so hard to come by