Onigiri is a real-life Japanese food that is a staple to Japanese culture, often seen in anime and video games. It is often simply referred to as "rice balls" outside of Japan.
Notably, a sentient Onigiri appears as an enemy in Wrecking Crew '98. It is a living rice ball snack which can float through the air and jump very high. When it gets upset, it often explodes. Onigiri is the first secret opponent Mario encounters in the game.
Outside from Wrecking Crew '98, onigiri have appeared in other Mario-related titles, including being a Food item in Super Smash Bros. Melee, Super Smash Bros. for Nintendo 3DS / Wii U and Super Smash Bros. Ultimate, and being the focal point of the Rice Stuffed microgame in WarioWare: D.I.Y.. Mad Scienstein can be seen eating an onigiri in The Wario Hop minigame of Wario Land 4.