This is the KRE-O 10162, which first appeared in a blue version from the early version of KRE-O Transformers Optimus Prime Dino Hauler, working as a ball for the midsection of the truck. Sadly, this has later been altered to be black.
White and gray versions can be found below:
As far as I can tell, each arch half is made out of:
40379 Dinosaur tail end section
28870 Plant stem curved
4733 Brick 1x1 with studs in four sides
15208 Plate 1x2 with 3 teeth
49668 Plate 1x1 with tooth
37762 Cylinder 1x1.333
2x 43888 Brick 1x1x6 round with square base
35480 Plate 1x2 with round ends
There might be a 25269 1x1 round tile, quarter between ...
This is one of Eastern Germany's (DDR) two brick systems (created by typical VEBs and I'd be suprised if you find those letters anywhere printed on the bricks):
most likely Formo because of the simplicity of the backside and how basic all the bricks are - you should not be able to combine it with Lego bricks and these bricks are rather difficult to ...