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.
The main emulators off the top of my head, MAME, bsnes (and the related higan) and dolphin, are all in C++.