This is a version of the “turtle” aperiodic monotile.

It has the most helpful turtle trace area in the center, but that's not all!
It can be really hard to draw aperiodic tiling without running into collisions or gaps. This stencil has a grouping of 8 that is really easy to trace, and is much simpler to generate large infinite tilings with! You can see the points of the “turtle” head and neck to align everything well.

Based on the work done here: https://discourse.mcneel.com/t/an-aperiodic-monotile/156830/68?page=4