That it was (a separate chip), but what is perhaps less well known is that Intel also produced an I/O co-processor chip called the 8089. That chip was interesting to write code for, as it tried to offload various I/O operations. At the time, Intel was on something of a "systems" tear, building their own small systems to compete with other computer vendors like DEC, Motorola, and TI.
The 8089 was a total flop relative to its development cost and Andy Grove declared Intel would not do any more Graphics or I/O processor chips. (I was the Systems Validation Engineer on the 80782 at the time, so much for my project!) As it turned out I think it was just too early for a specialized co-processor.
I wouldn't say too early. There were a fair number of temporarily successful co-processors in that general era of many different sorts; aside from float (and smaller markets for FFT chips and boards and some efforts at vector co-processors), the most famous were probably the multimedia coprocessors on the Atari ST and on the Amiga (Jay Miner etc).
If that's too late for your tastes, look at the nearly universal support chips for 8080/z80 like DMA. Possibly one could count UART and PIO too.
But Moore's Law kept killing them, and eventually it became common wisdom that such was the case, so they faded.
Over a long time period, external devices to offload the primary cpu come and go, under different guises, as Ivan Sutherland noticed already in 1968.
Intel's 8089 and related may have been partially motivated by the success of IO processors in the mainframe and supercomputer world in the 1960s and later.
On the 486, that chip was actually a full blown CPU there was no 487 co-processor. Instead, Intel sold a full 486DX chip labeled as 486 that disabled and replaced the already mounted 486SX. (ref: https://en.wikipedia.org/wiki/Floating-point_unit#Add-on_FPU...)
