Mame 2003 doesn’t show up in any list. I can find all other cores, some are in the “experimental” list but 2003 is nowhere to be found.
Why are you asking here ? We don’t manage the retropie distro, as far as the libretro team is concerned, the mame2003 repo is still there (https://github.com/libretro/mame2003-libretro).