The original AMD 8088 goes back to the days when it was routine for manufacturers of electronic products to insist on second sources being available. It was a true second source of Intel's chip, based on the same die design but made in different foundries. AMD's 80286 was also based on Intel's design. Intel then decided not to share later designs with AMD, so all their later x86 chips were at least partly designed by AMD.
AMD's first fully in-house design was the K5, but it and the follow-on K6 preserved pin compatibility with the original Pentium. The K7 (Athlon) was the first one to diverge farther from Intel's chips; it was not pin-compatible and required its own motherboard design. The K8 (Athlon 64) was the first chip with the x86-64 instruction set extension that was later also adopted by Intel.