This is Mario & Luigi, an un-official Mario game for MS-DOS released in 1994.
To play this offline, you need DOS-Box (x64 Windows/non-Windows users) or NTVDM (x86 Windows only) Also, for everyone, first set up the Rotating Screen controls, or else you could accidentally flip your screen!
Arrow Keys: Move
Space: Throw Fireballs (Fire Mario only)
This game is here in the Internet Archive for you to...