Spot Mini is an amazing (perhaps scary) robot dog built by Boston Dynamics--not for sale.

If you want to purchase something similar, a company in China produces a research platform for around $30,000. Another legged platform, Anymal, can follow you into an elevator.

Using a 3d printer, off the shelf servo motors, an Arduino and a couple of ultracapacitors; I have started something along the same line--but the cost is more like $300. 

Don't get too excited--my dog doesn't walk or stand yet, but it can lift its own weight and I have overcome a couple of challenges.

If you want to use the 3d print files (and some of the design files) go to thingiverse.com. Some of my .stl files do not have a corresponding design file because the .stl files are 1) the combination of two or more .stl files in Tinkercad or 2) they are mirror images of .stl files, mirrored in Tinkercad. The leg pieces can take 10-12 hours each to print as they are sizable.

I use twelve high torque (277 oz.in) metal gear servo motors at a cost of about $18. each. These have no end stops and they draw a fair amount of current to move, but they respond consistently to commands.

Under heavy load, each servo motor draws 3.5 to 4.5 amps. To move from a sitting to upright position, eight motors need to provide significant torque--meaning 30 to 40 amps are required for a second or so. I use three 350 farad ultracapacitors (in series) across my motor power supply and they take care of the momentary load of the servo motors.

My first "shoulder connection" put all the "up/down" and "rotate" stress on the servo motor.

This looked like a bad idea, so I created a bearing block and heavy shaft to absorb the "up/down" load and let the servo motor's gearing handle the rotation only.

The bearings are snowmobile bearings available on Amazon.

I designed the servo motor mounts such that there is a sort of "back bearing" opposite the shaft so that it is easier to attach moving limbs.

The first foot was a wheel--but that rolled all over the place, so I created a movable flat pad for better traction.

I mounted a digital voltmeter up on the body--just to keep track of what was available for the servo motors.

So, there's a long way to go, but this walkable platform (when it works) will be useful for robotics on smooth or rough terrain.