I've collected a list of organisms that could easily yield easy-to-draw descendants, for the benefit for newcomers who may have limited art skills. This list is not comprehensive. I'm posting it here for feedback or expansion.
If your art skills, science skills, and writing skills are all very limited, the easiest possible option is selecting a genus group of aquatic flora that is easy to draw, summarize some details from the genus group description with a few bits of novel information (possibly paraphrased from pages on round algae species, like Neptune's necklace, or kelp), and place its habitat in a temperate or tropical coast habitat.
General Principles
When in doubt, draw from the side, without background details.
Deep sea fauna are more likely to have thick, simple body shapes than those in shallower water, so you can look for organisms in Twilight Floor and habitats and lower, or even make a descendant for a deep-sea organism that’s even simpler.
If a slender-bodied organism takes on a burrowing life, its limbs may become smaller, or it may lose them entirely. You can use this to make simpler, easier-to-draw descendants of an organism with a compelling in-universe explanation.
Usually, official art of organisms shouldn’t be drawn from above, but if the organism in question is flat, like a flatfish, and doesn’t really have anything remarkable on its underside, it can be drawn from above. This is true for Flat Swarmer descendants, and they’re easiest to draw from this angle.
Organisms tend to get simpler body shapes (e.g., lose limbs) if they become endoparasites, although with the limited number and diversity of parasites on Sagan 4, it might be difficult to find good candidates for fauna that can develop into endoparasites.
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Total Beginner
If you can't draw anything more complicated than a flat smiling sun or a stick figure, these are good choices.
Just Circles
If you can draw a circle/sphere, color it solid purple, and give it shading or waves or circle patterns (so it doesn't look very lazy, even given your limited art skills), you can draw a Marbleflora. This is also extremely easy to do in digital modeling. There are many guides for shading spheres online.
Marbleflora
Sanguine O’Spheres (can be depicted excised from host skin, or against a solid background the color of the host's skin)
Chambered Bobiiro (patterns)
More Complicated:
Dry Gelatin (mostly a sphere, but with wrinkly, leathery skin and pores)
Circles Plus Other Circles
Ouhciiro (circle connected to a smaller circle)
Sheet Snotflora (circle within a circle, with little stringy pseudopods within the inner circle)
Cloudbubble (sphere covered in hair)
Parasitic Floats (Circles connected with strings, with strings at each end of the chain)
A Circle With A Little Extra
Mostly a circle, with small non-circle parts or patterns that can be portrayed as flat.
Twinkiiros (one shape is a sphere on top of a cylindrical shape, like a can)
Pioneeroots
Omegiiro
Lediiro
Other Basic Shapes
Bonecorus (a heart, ridges, and strings at the end)
Otter Salentoid (football with little roots)
Smears and Blobs
Microbes
Salmound (irregular blob. Segmented like an orange cut in half.)
Flora
Flashkelps
Swarmerweeds
Basilliphyta (flat blobby shape on a rock)
Bonyfee (colorful gumdrops or other rounded shapes; just draw on a bone fragment to make it easier)
Slarti Glasstower (Gumdrop with a blob)
Fauna
Foi are, in general, very easy to draw. But when writing a description, remember that they're very, very simple, more like placozoans or slime molds than "normal" animals.
Examples: River Foi, Dixon Foi, Camouflage Foi, Minifee, Water Table Foi, note that some have a rim around the edge)
Flange-Nosed Foi (a smear with two rounded triangles on one end)
Mortusyte (tapered, oscillating smear, with a pointy shark-fin triangle for a fin)
Kuyasha (Make a fuzzy blob with a coarse-edged paintbrush, paint in some slight variations of grey with the same brush, then add a star with a circle inside it for a close-up view)
More complicated smears and blobs:
Lactic Foi (smear shape, core, transparent, leech-like rim)
Mortuprey
Organized Blobs/Round Shapes Plus Other Shapes
Microbes
Morphous Swampbean (various simple shapes)
Beach Bean (a lot of circles and blobs)
Flora
Larands (Larands genus group, Larachoy, Larandbora)
Fuzzpile (Blobby smears stacked on top of each other, with long hair and many circular berries)
More complex:
Kack Tower (a somewhat pinched-in, upright smear, covered in spikes and cactus pleats, with spiky fluff on top)
Coralkiiro (a sphere with triangles)
Sappro (from a distance: a blob covering flora; from close up: basically a row of teeth in their sockets.)
Fauna
Keryhs Clade
These are tiny, wobbly endoparasites that come in a variety of forms. If you can draw a tooth in its socket, you can draw these.
-Keryhs (genus group)
-Fang Keryh (wobby stalagmite shape with two tentacles)
-Pricklecone Keryh (wobbly stalagmite shape, four tentacles, tiny triangles at the tip of the big triangle; larva is shaped like the head of a wrench with four tentacles on top)
Qural Clade
-Qural (half a big circle, a squarish shape, and tentacles on top)
-Artir Cural: (a very thick pill shape, with a smear below, and lots of tentacles on top)
-Jaydoh Cural: A wobbly, elongated pill shape, with a square at the bottom, some black ovals for vents, blotchy colors, and tentacles at the top).
Swarmers
Not all swarmers are easy to draw, but a fairly large number are.
-Miniswarmer (especially the smallest, roundest sample)
-Deep Glowswarmer
-Flabnose
-Flatswarmer
-Ray Flat Swarmer
Vaguely Animal-Like Blobby Shapes
If your animal drawings look like fused-together chicken nuggets, these are good for you.
Topyiiro (variable blobby shapes)
Lureflab
Rojoko
More complicated:
Stegosnaper: Big head, gills, thick tube body, short, thick tube-limbs, blobs of scutes, blobs of a mouth, no eyes)
Scooters (tend to have rounded shapes; can use spheres for the torso, eye, thighs, and triangles for the back spikes.)
Basic Non-Tube Fauna
Aphluks (If you can draw a snowman made of two balls of snow with bunny ears and three twigs with three branches each, you can probably draw this)
Gilltails (elongated football shape, with tail fins like hearts with triangles on top)
Snakes & Worms
Limblesses (somewhat similar to snakes; the mouths are complicated, but you can simplify them for a descendant)
Cleaner Borevermid (segmentation, spikes, somewhat complicated mouth; combine smear, rectangle, circles, triangles, and oval)
False Cleaner Borevermid (similar to Cleaner Borevermid, but more segmentation, and a triangle mouth with more segmentation)
Slightly More Skilled
If you can draw a roast chicken, you can probably draw these.
Geometric Shapes
Shapes which are simple to draw, but not round or blobby.
Kory Claw (bending pyramid, attached to a semicircle; three colors)
Terrace Crystal: Soil, long rectangular box, triangular prism, rectangular box, triangular prism.
Pagoda Crystal:
Soil, Rectangle, Four trapezoids, one triangle. Split evenly to indicate shading. Ideally, put a little shading underneath the edge of each, to indicate depth. Look at Egyptian pyramids for shading cues.
Crystalfir/Emeraldfir :
A box, three progressively narrower boxes stacked on top of each other, and a triangular prism on top. Small spheres just underneath the edge of each row. If you can draw a simplified Christmas tree, you can draw this.
Phantasmagoria Crystal (long rectangular prisms (boxes) with pyramids on top.
Signpost Crystamboo: long tube (like a paper towel roll) with segmentation (like cans stacked on top of each other), and skinny pyramids between segments.
Vaguely Plant-Like Simple Shapes
If you can draw a recognizable piece of broccoli, you can probably draw these.
Pert
Pioneer Quillprong
Fuzzyfan
Piomike
Cushprongs
Kellace (circle/blob, curving stalks, tiny dots. Optional: wandering roots in a cutaway of a crystal flora)
Colonial Calmstrum (if you can draw a generic, simplified daisy-style flower, you can draw this)
Simple:
Sapshrooms (half an oval, tilted on its side; stalks, and tiny circles at the end)