Spike Tops are "primitive" types of Koopas that would appear to be related to Spinies and Buzzy Beetles. In fact, they could be said to be a cross between the two, as they have Spiny's white spike and general coloration and Buzzy Beetle's hard, round shell. An oddity of Super Mario World's Spike Top sprite is that it has six legs, like an insect, instead of the four that would be expected from a turtle creature. In later appearances, it has the expected four legs. This is assumed to be because the people in charge of making the sprites were told it's a Buzzy Beetle, and since a beetle has six legs in real life, it was given six legs in the game. However, that doesn't explain why one of its animation frames has only four legs...
Spike Tops live underground and slowly shuffle along. When they come to wall, instead of turning around, they climb right up. They're even able to walk on ceilings, an ability that Buzzy Beetles and Spinies had in Super Mario Bros. 3, but lost for Super Mario World.
Spike Tops are immune to fireballs, but cape swipes, thrown objects, and stars will defeat them. Yoshi can eat them too.
Spike Tops first appeared in the level Donut Plains 2.
Spike Tops are sprite 2E. They use the red palette C. Spike Tops can only be used with the underground sprite set (SP4=GFX04).
Spike Tops have difficulty navigating slopes, so make sure that surfaces they're able to climb are made of 90° angles. Spike Tops will only appear if Mario comes from the left to right, so they're incompatible with left-scrolling levels. Plan around that. However, this could be useful for having a situation where Mario has to go to the left extremity of an area and return right, so that "it's tougher getting back."