Thwomps (the enemies from Alucard648's Sideview Engine) and icicles edited from Super Mario Maker 2 for Classic. Make sure that there's two combos for each, one with the closed eyes and one with the open eyes, that each combo has the upper-left most part of the Thwomp as its graphic, set the enemy's Misc. Attribute 11 to the first combo number of the two, and finally set Misc. Attributes 3 and 4 to the correct numbers depending on how big it is.
The Thwomps come in CSet 11 (grey) and CSet 9 (level-dependent) versions, and the Bomb Thwimps and Falling Icicles use CSet 7, the Super Bomb Thwomps use CSet 8, and the non-falling Icicles use CSet 2.

Types:

Thwomps (have 4 directions)
1x1 (Thwimp)
2x1
1x2
2x2

Flat Thwomps (only 1 direction because you can't stand on any other side) (these don't have a script yet)
1x1 (Thwimp)
2x1
1x2
2x2

Block Thwomps (4 directions - once they are triggered and hit a solid combo they turn into gravity-affected pushable blocks) (these don't have a script yet)
1x1 (Thwimp)
2x1
1x2
2x2

Bomb Thwimp (4 directions - once it hits a solid combo it makes a bomb explosion that can also break cracked rocks)
Super Bomb Thwomp (4 directions - once it hits a solid combo it makes a super bomb explosion that can break cracked rocks in a bigger radius, or tougher cracked rocks)

Falling Icicles (Meant to be Thwomps that either break when they hit the ground and turn into shards like in Super Mario Maker 2, or can also turn into solid combos like the big ones in New Super Mario Bros. Wii. We're still waiting on 2.55 to have support for scripts spawning grass shards)
1x1
1x2
2x2

Combo Icicles - These are just solid, unmoving (and possibly damaging) combos that use the level colour. They are like the icicles in Super Mario Maker 2 that don't fall and can be used as decoration without scripts.

Nintendo