This is a stopper knot that is somewhat bulkier than Ashley’s Stopper Knot and is derived from it. Where Ashley’s shows a trefoil pattern when looking down the standing end, this knot has a four-fold pattern around the standing end.
Topologically the knot is a double overhand noose with a tuck back.
It is easiest to tie without the tuck at first. Tighten up the double overhand knot around the standing end by eliminating the A segment. This will bring the B and C legs together with the standing end inside the loop. Snug this up enough that the standing end is no longer free to slide and none of the sub-loops are loose. The tighter this is made, the more secure the finished stopper.
Then, continuing in the same wrap direction around the B segment, tuck the working end through the noose as shown. Pull the standing end and the working end tight to create the four-fold pattern. In addition to being bulkier, if this knot comes loose there is a remaining overhand knot as a backup stopper.
If this knot already exists, please let me know so I can give proper credit.