What have you tried for finding the sprites?
Opening the ROM in a tile editor, setting to the appropriate console mode and pressing down/page down a lot might well get you what you want.
If you know where something else (if this is a rare enemy then probably with other enemies rather than megaman and the robot masters, though it can be all in one) is then chances are other things will be nearby somewhere.
If you can make it appear in the game then you can try things.
Simple one is find it in VRAM and then search the game for that data (or a fraction of it). Compression will get in the way of this but such things are not the most common on such devices (though if it was going to be one then megaman would be up on the list).
Typically such things will be loaded either at the time a new "screen" is loaded or when the level is loaded.
Tracing is probably the next step. As mentioned above then most consoles won't tend to stream things directly to video from the cart and will instead copy something and go from there. The NES is a potential exception here as it does feature something called CHR ROM and CHR RAM, though that actually makes it even easier https://wiki.nesdev.com/w/index.php/CHR_ROM_vs._CHR_RAMhttps://wiki.nesdev.com/w/index.php/PPU_memory_map
Get a debugging emulator and set a break on write (called different things in different emulators but probably bpw here in short) just before it is loaded in. Whatever gets used to shuffle data around (can vary by system, though usually limited to something the CPU does, DMA or something more specialist for the system in question) then should have a read type (might be important), source and destination.