The Super FX co-processor was, according to Wikipedia and every other source I can find, a custom designed RISC chip made by Argonaut Games.
Argonaut was a small British company and back in the late 80s developing a custom RISC CPU was a very, very complex task. RISC was fairly new, and high speed CPU design (the Super FX ran at 21 MHz, pretty fast for the time) was far from trivial.
Did Argonaut do all the work themselves? Conceptual design, electrical design, internal layout, testing, certification for EMC etc. Or was some other company involved? Often such work is farmed out, based on a design document, to an ASIC design specialist.
For example, has anyone decaped a Super FX chip to see if there are initials or logos on the die? Is the RISC CPU based on some other design, as for example the SNES CPU was based on the 65C816.