This design was inspired by this awesome sea turtle model that I saw on Thingiverse. I wondered what other animal I could make move in a way that could be powered by a person turning a crank. I thought that a majestic 2 inch tall horse, galloping in place on somebody's desk would be pretty cool, so I got to work.

Supplies

Computer
3D Printer (Optional)
3D Printer Filament (Not optional)
Glue

Step 1: Concept

First of all, I had to figure out the correct movement of the horse's legs. I started off by watching slow-motion videos of horses running so that I could get a good idea of the hinges and movements involved. I found that the front and back knees bend in opposite directions, which created an extra challenge for me. I had to come up with a mechanical system that could hopefully be easily modified bend in the right direction.

To do this, I brainstormed possible designs, and tested them using LEGO™ Technic parts. Eventually, I found a mechanism that worked, and I started to design it.

Step 2: Design

In a nutshell, the design process only required taking the basic mechanism from the Concept step and making it into a 3D Tinkercad design. I started out with an extremely simple gear layout, with only six gears to get the movement that I wanted. I designed simple struts that would attach to the gears and the horse's ankles, then I put pinholes in everything. The basic design looked pretty bland until I added the base, which is a mountain with space cut out to fit the frame with the gears. Finally, I took a model of a horse and cut it up into moving parts, and created axles for all of the hinges. This whole process took about 8 hours.

I printed out rough versions of all the base parts to work out any issues that might come up, and there was only one major (and stupid) one. I realized that the way I set up the gears would cause the back legs to move in the wrong direction. Luckily, I only had to change two parts: the frame and a single gear. After that, I printed out the horse.

Step 3: Print

All of these parts can be printed well at a slow speed with a .2mm layer height (except for Horse_Parts.stl, which should be printed at .1mm layer height). Horse_Parts and Horse_Body should both be printed with supports. This model requires a moderately precise printer and high-quality filament (I have a Creality Ender 3), so I made Printer_Test.stl, which contains a basic model with all of the connections used in the build, and it only uses about 1.15 cm³ of filament.

Step 4: Build

This is the most difficult (and interesting) part of this whole project. I would suggest watching the attached video and pausing it along the way. I have also attached a handy picture with the parts all labeled. To go along with those, here is a step-by-step process to build the model:
Once all parts are printed, make sure they are cleaned up so that no stray filament will interfere with the moving parts.
Construct the moving base.
1. Place the gears in the frame so that they all fit together (then remove the one with the long axle).
  - The gears go on the side of the frame that has text.
  - When permanently attaching the gears, it is very important that the holes are lined up so that they are at the top and bottom of the rotation at the same time.
2. Attach the disks with the holes at a 45-degree angle from the holes on the gears.
  - Make sure that the holes on the disks also line up at the top and bottom of the rotation, 45 degrees off from the gear rotation.
  - Each pair of disks should be rotated the 45 degrees in the opposite direction, as shown in the video.
3. Use the tiny pegs to secure the long pieces onto the disks and gears as shown in the video.
  - Those pieces, shown in white in all of the photos, have a short side and a long side. The long side should be the one sticking up above the frame.
4. Use whatever means necessary to secure the frame to the base so that the empty holes in both line up (I used glue) with the gears on the side where they are exposed the most.
5. Put the small gear with the long axle through both holes on the frame on the base.
  - IMPORTANT: rotate each pair of gears so that the long pieces are close (but not too close) to the same position.
6. Once the small gear has been fitted into the other ones, glue the tiny crescent-shaped piece into the slot on the axle.
7. Glue the cylinder-like piece with the peg into the peg hole on the crank with the text in it.
8. Secure the crank onto the axle sticking through the hole in the base.
Construct the horse
1. Place the two hammer-shaped axle pieces in the circle holes in a half of the body, with the short side on the right (a little bit of puzzling will tell you which side that is).
2. Glue the other half of the body onto that.
3. Separate the back leg parts from the front (more puzzling. Hint: the back knees bend forward). There are no axles for these, because I thought it would be cool to use 1.75mm filament for that.
  - To secure the filament in the knee joint, first put the filament in and cut it, then glue the outside.
4. Take the nail-shaped pieces and put them through the hammer-shaped pieces (from the long side on the left).
5. Put the legs on those axles.
  - When gluing these, make sure that they are not glued to the nail for the left legs, and the hammer for the right.
  - It is important to get the correct angle relative to the axle on the left legs. I would suggest not gluing those until the head and tail are attached.
6. Attach the head and tail (make sure they are able to rotate as far as the arm on the axle can).
7. Put the horse on the base by putting the larger pegs through the long pieces into the ankles.
It is very important that every joint is able to rotate very easily
Rotate the crank counter-clockwise to make the horse gallop

Note: There were a couple of issues that I ran into while I was building, and I fixed them in the 3D files. In hindsight, this whole thing isn't very structurally sound, and eventually I want to do a complete redesign to make it last longer and move better.