People here have had great descriptions, but I want to throw in my 2 cents, too.
Origins:
All of the ponies individual
parts were made in the factory (ie head, body, wings). All parts are identical to official ponies' parts. The hooves have the hoof stamp, and sometimes the manufacturing date, just like the official ponies, too. The heads have the same stamps on them as well, under their hair. That doesn't mean that the whole pony was assembled at the factory though. It is kind of a moral grey area because don't know if people are stealing them, getting them for free or if they take advantage of Hasbro, etc.
Types of ponies available:
Regular ponies - Most of the time, you will find completely normal ponies here, the same as what is sold in packages, except in bulk
Error ponies - Sometimes reject ponies with dirt on them, terrible hair, stains, or paint problems are sold
"Franken-ponies" - These are simply jumbled up ponies, where people either switched heads/tails or did not assemble the ponies properly. Most are just jumbled parts of existing releases.
Test ponies - A subset of the franken-ponies, occasionally, you will see ponies that have different hair, eyes, symbols, or parts that any existing ponies. The factory likely made these for testing purposes, and they were never used for any existing releases.
Prototypes - Very rarely, actual prototypes will pop up on Taobao. These are ponies that are clearly the earliest design of a new product, not just random combinations of parts.
Worth:
Errors (quality problems) - less than $5
Regular ponies - same as regular releases (most around $5)
Franken-ponies - Most of these will be worth around $5
Test ponies (subset of Franken-ponies) - anywhere from $5 to $50
Prototypes - generally fetch beyond $50, with the most popular, Minty probably being worth several hundred dollars.
TL;DR, you can sometimes find ponies on Taobao that are not available anywhere else.
Taobao is pretty much Chinese Ebay... Most of the ponies from there are prototypes and franken ponies
The thing that gets to me, they are sold with no provenance. They come with no COA (certificate of authenticity) which means they will down the road not hold there value. I have seen some fun ponies in Brags but that worries me a great deal.
Yeah most of those are only worth $5 and are just fun to collect because they are oddities. People are using the word "prototype" for everything, too, but a lot of the ponies are just scrambled up ponies.
I am guessing the actual prototypes will probably hold their value though, since they are unique from existing ponies, have hoof stamps, date stamps, etc, and would be too hard to be faked outside the factory.
I am curious though, do any ponies ever come with certificates of authenticity? Who would issue that sort of thing?