A CPU in the 80486 family developed by Intel and released in April of 1991.
Clocking in at a measly 20MHz, it has the dual distinction of being a member of the FPU crippled 486SX line as well as being one of the slowest Intel 486 processors ever on the market. Only the just as rare 80486SX/16, released 5 months later, was slower.
Like all 486s, the CPU was multiplier locked—meaning that the CPU ran at whatever speed the front side bus was set to due to its 1x multiplier. There are unconfirmed reports of the CPU being successfully overclocked up to 33MHz by playing with this bus speed.
The CPU itself would probably be more infamous but it was notoriously hard to find and as such is overshadowed by its marginally faster and vastly more common sibling the 80486SX/25, which was essentially the same CPU on a 25MHz bus.
Given the choice between using a 486SX/20 and a 80386, I'd take the 386.