I like how Find Familiar works in the 5e PHB, with a celestial, fey, or fiendish nature according to the nature of the mage who summons the familiar. However, I’d like to vary things up a little bit more, so here are some not-so-familiar alternatives to how to “skin” your familiar. Like with the familiar options in the books, the appearance doesn’t change your familiar’s abilities or traits; it just gives them a different flavor.
1. Abstract- the familiar is an abstract entity, either appearing as a drawing or a living sentence or equation of such complexity forms the creature.
2. Blood creature- the familiar is summoned through perverse blood sacrifice; it continually dribbles blood to the ground. The caster must slice open his own flesh (doing no damage) to call the familiar.
3. Construct- The familiar seems to have an obvious artificial or mechanical nature. Clockwork familiars are a favored design here. Another favored alternative is for the familiar to be made of paper, much as the shikigami.
4. Corpse creature- either skeletal, zombie, or ghoulish. The spellcaster doesn’t conjure the familiar so much as pull its festering form from the ground.
5. Diseased- the familiar seems unnaturally diseased. It may be pox-ridden, twitch unnaturally, molt uncontrollably, etc.
6. Elemental- Pick one or a combination of the elements that the familiar is made of.
7. Fleshwarped- the familiar’s body has become warped or mutated in some fashion.
8. Gaunt or Obese- the creature seems either unnaturally gaunt and frail or unnaturally bloated and obese.
9. Gelatinous- the familiar is made of ooze, slime, or jelly.
10. Ghostly- the familiar gives off an eerie glow and appears almost insubstantial or translucent.
11. Obnoxious Hat- The familiar appears normal but wears a gaudy, obnoxious, and/or unsightly hat. (Roll on an appropriate table or just come up with the most obnoxious thing you can think of.)
12. Personal Avatar- the familiar bears an unnatural resemblance to the spellcaster, going up to having a tiny freakish human(oid) head instead of a normal animal head for its species.
13. Plantform- the familiar is not so much an animal but a mass of plant or fungal life emulating an animal.
14. Pirate- the familiar appears quite pirate like. It might have a bandana, an eye-patch, or even a peg-leg. Regardless, it seems pretty rough and tumble. Other variant “costumes” might be possible, such as ninja, cowboy, Viking, etc.
15. Pseudonatural- a hideous, tentacled, slathering mockery of life marked as one of the Far Realms, the Dungeon Dimensions, or other dimensions better left forgotten.
16. Runecarved- glowing, magical sigils or tattoos have been etched into the creature’s flesh, fur, scales, or feathers.
17. Shadowform- ripped from the spellcaster’s own shadow, the familiar appears to be made of the umbral substance that makes up the Plane of Shadow. The familiar may even appear flat/2-D like a true shadow.
18. Swarmform- The familiar is not made of a single creature but of many tinier creatures that form a single solid creature. These could be miniature examples of the base familiar, or of smaller creatures such as bugs, worms, maggots, etc.
19. Tumor- the creature is not so much a separate creature but a tumorous growth taken from the caster’s own body. Each time the familiar is dismissed or recalled, it joins or splits off of the caster’s body.
20. Uncanny Valley- the familiar appears more or less normal, but there’s just something quite noticeably off. Perhaps it’s the way it gazes at things or it moves totally unlike a living thing should. It needn’t be obviously malicious to qualify- perhaps it’s too sweet or cute to be natural…