From SMWiki

A Mecha-Koopa is a mechanical Koopa from Super Mario World. It only appears in two levels: the "FRONT DOOR" of Bowser's Castle, and the "BACK DOOR". It was also used as a projectile; it was thrown in pairs by Bowser to attack Mario, but his plan backfired horribly when Mario stunned the Mecha-Koopas and threw them back at Bowser.

[edit] General Information

In the original game these were quite rare, so you didn't have much of a choice to attack. Most attacks will only stun him, but a star should take him out in one hit. Other than that, you could call them invincible, or at least, not killable with Mario. They are killed when touching the below side of Bowser's Koopa Clown car, and possibly - but never shown in the original game, and it would make up for graphical glitches - also by Bob-Ombs. They might be edible. Their attack pattern is quite predictable, but in Bowser's fight, it might happen at moments you don't want it to. If you stomp them, they're stunned, but if they return in the heat of the battle, they could be an annoyance and if you hold them and don't notice they're returning to life, they will even succesfully attack you. Always have an eye on them.