Classic Piranha Plant
A Classic Piranha Plant is an unfinished sprite in Super Mario World that acts just like regular Piranha Plants in previous Mario games. Classic Piranha Plants tend to live in pipes. They extend and retract at regular intervals, trying to dig in with their sharp teeth. Oddly enough, they won't show themselves if Mario is within a one square to either side of them. In order for it to show up properly, you must use ExGFX to overwrite the Flopping Cheep Cheep graphics that are used for its stem by default and change the palette of the stem using Tweaker. Beware, though: without the Classic Piranha Plant Fix patch (also included in this patch), the sprite will not work quite correctly because of an obscure VRAM error. In Super Mario World, however, there is also a Jumpin' Piranha Plant.
The glitchiness of the classic piranha plant is not purely aesthetic. Very rarely, if you have a classic piranha plant in a level, another sprite tile on the same screen of the plant may get repainted, and may, in some circumstances, jump around between SP1/SP2 or SP3/SP4 and/or get flipped around. This is why a simple graphics change in YY-CHR of the plant to make it LOOK correct is not quite enough to actually fix it. The Classic Piranha Plant Fix patch fixes this error.