Shikigami are conjured beings, Which means they are born for the energy or ki of the magician. Shikigami are also a spiritual extension of ones astral or higher-self, with it having its own but similar opinions and agenda to the practitioner. Familiar spirits or Shikigami in Onmyoudou are seen as tethered or bound beings used to interact through the astral world. The spiritual tether that holds a Shikigami is not symbolic to a physical one, but it is a magnetic or energy over the conjured spirit. Therefore the shikigami is created from and still is your ki and or energy you have control over it through mental, subconscious, and spiritual thought and or will. To reiterate the extension or tether is the power over the being, not a spiritual cord.
Whats do Shikigami look like? Shikigami look as if they were made of smoke. Meaning they are energy being made from particle or orbs or aura or spirit. They may range from their true form of a mass of wispy aura and energy, to shapes surrounding the creation of an object, person, or animal. Shikigami are shape shifting spirits but are able to take the form of certain animals depending on the symbolism of the magician's energy.
Can you have more than one Shikigami? Yes. There is no limit to how many spirit a person may conjure.
What are their powers in the occult? Shikigami do not have powers only spiritual abilities such as possession, depending on the animal form it takes the Shikigami is able to gain the animal's spiritual aspects. Possession is the state of having, owning, or having influence over something, combined with the Shikigam is unique ability to take the form of people, animals, or objects in nature it is able to gain magical correspondences and aspects of nature itself.
How do I care for one? Shikigami, do not like to be alone in the astral. They enjoy the warmth of a physical spirit vessel or a talisman. You may care for this being by creating a talisman linked to the energy gathered by your altar and allowing the Shikigami to live there.
Are they dangerous? Yes, but so is everything else that is abused or disrespected. If a Shikigami is offended it may choose not to aid in your spellcraft. Shikigami do not lose control of emotions very easily and are very kind and bright minded. If you the magician choose to force the Shikigami through incantation or ritual to act on your behalf, then it may cause misfortune and so on.
What will me Shikigamis animal form look like? I do not know. I am unable to track that. I can just tell you may know through visions. When I was a boy I loved foxes, I read stories, about them and everything. I did this because the felt relatable and I felt akin to them. later in life, I had a vision of a Red fox in the snow during the 4th period of junior high. So you see you may know through intuition or visions and dreams.