redream is a Dreamcast emulator written in C only. libretro (the core API behind Retroarch) and Retroarch itself are primarily C. mupen64plus is in C, although the common 3rd party plugins and frontends are not always in C.

qemu, a dozen gameboy emulators, dosbox, the 9front emulators (NES, SNES, GB, GBA, C64, 2600), xemu, z80e, and many, many more. Lots of the C++ emulators you're familiar with have the performance critical parts written in C as well, excepting some newer emulators which leverage something like LLVM for JIT emulation.