A unicorn. A four-legged horse/unicorn/pegasus.
If I can get the base avatar body morphed into a shape with arms and legs roughly equal in length, I want to go with custom animations and an AO for most of it. Blinking and such can be prim animations, and I'll probably use flexi prims for hair.
The base avatar can take a shape close enough to what I want. Long torso, long arms, short legs, long neck, tiny hands and feet. Extremely thin and unmuscled - it will be covered with prims, so the less of it the better.
Next is an animation of a horse standing pose. I use QAvimator, but use whatever you're comfortable with. I make the pose looping and priority four. Like with Tinies, a quadruped avatar needs its animations to be priority four.
I stick the animation in a pose stand so that I don't have to fuss with using it. This lets me "build" the prims around the avatar without having to worry about it moving.
Prim wrangling goes well. I've got pictures of horses to compare with while I build. If you're placing prims around joints, it is important to be thinking about how those joints will move once the avatar is done... I tend to put spheres over joints. When the prim sculpting is done, I take a copy of it into inventory as a backup.
Next I break the sculpt up into attachments and fit them to the base avatar. I do the linking and fitting of one attachment set at a time. This lets me use the original sculpt as a reference when placing each attachment. I'm not worried about leaving the skull attachment point free ( I wouldn't expect people to add prim hair to this avatar) but I will leave the spine free in case someone wants to add wings. I'm also leaving the horn as its own attachment so that people can swap it out for other horns. When the entire things has been made wearable, I take a copy of it into inventory as a backup.
Time to make more animations. Galloping, a walk, a sit and a ground sit, a couple more poses... The internet has plenty of pictures of horses in natural poses, and videos and stills showing different paces. I load the AO and decide to make it a separate attachment.
I watch the unicorn move and adjust the prims around any joints that expose part of the basic avatar during an animation. Another trick for that is to make the base avatar's skin match the prims. I usually do both.
I script blinking eyes, moving nostrils, and swiveling ears.
I go back to the head and break it up a bit. The mane and the eyes are now separate attachments. Along with making the tail modifiable, this lets me set the parts most people customize as mod while protecting the basic shape and proportions so the animations work well. I make a couple sets of eyes with different textures.
In order to sell it, I check and set permissions and box it up. I spend a couple minutes in front of a fullbright prim taking screenshots and use the results to make an advertising texture.