This is my improved version of the classic Flexi Rex. Somehow this version became the most popular one, maybe just because it doesn't break easily when handled by kids (and adults of course).
This model has existed since December 2017, but I uploaded it to MakerWorld as well for everyone's convenience. The primary source for this model remains its GitHub repository where you can find the latest updates, the model's history, and detailed instructions.

Go to my site for an overview of my other models.

Attribution:

This is a remix of Kirbs' Flexi Rex (Thingiverse thing:1759297), based on airfish's remix (Thingiverse thing:2189652). That model in turn was most likely inspired by the ‘Flexy Rex’ by zheng3 (Thingiverse thing:929413).

The reason why I made this version is that my print of Kirbs' original model was not very kid-proof: one of the links broke after some rough handling. Hence I made all links more robust. This means this model can probably be printed even smaller than the original ones before the links become too weak. Of course you can also scale it up at will.

I have further tweaked the model to be more balanced: it should be less inclined to face-plant than my previous version or other remixes of Kirbs' original model. Also, balancing the model on its tail should be easier, although success still depends on your particular print settings and accuracy.
If you want to print this larger than your printer allows and don't mind some manual assembly, try the separate parts version.

Note to anyone wanting to sell this:

You can sell prints of this model without having to ask explicit permission, but you must honour the Creative Commons - Attribution - Share Alike license. See the GitHub page for detailed instructions.

(And yes, if you see other copies of this model on this site or others that do not refer to this original, either through the GitHub page, the old Thingiverse page, Printables, or this very page, then they are violating the CC-BY-SA license indeed. But since I'm not the author of the truly first main design either, I won't make a fuss about it and trust that everyone will eventually update their copies with a link to the original source. If you notice a remix without the required attribution, you can help me out by asking the author to update their model description—in a friendly way).

If you want to remix this:

If you publish your remix, all you need to do is mark it as a remix and select this model (297490), and give your remix the same Attribution-ShareAlike license. If you publish it elsewhere, follow the instructions from the GitHub page.
(Mind that this has been remixed so many times that you might have a hard time doing something that has not already been done.)

Print instructions

The recommended print settings are:

• 0.2 mm layer height
• 3 perimeters (important, to make the links strong)
• 15% infill

Expect about 2 to 2.5 hours print time for a good quality print.

Don't use a high infill value, this could cause it to become unbalanced. For improved stability, you could increase infill on the piece with the feet below it, and reduce infill of the head, but this should no longer be necessary if you print the current model with the above recommendations.