List of Enemies

Enemies do 2x damage in Hard mode, and 0.5x (rounded down) on Easy mode.

Varia Suit reduces damage by 20% (rounded down) and Gravity Suit reduces damage by 50% (rounded down).

Enemy	Name	Damage (P/V/G)	Health	Power Bombs	Supers	Missiles	Large Health	Small Health	Nothing	Notes
	Acid Worm	10/8/5	600	4.88%	4.88%	21.88%	1.95%	29.30%	37.11%
	Acid Worm Spit	10/8/5	1	0.00%	0.00%	29.30%	21.88%	39.06%	9.77%
	Atomic	30/24/15	40	0.00%	0.00%	0.00%	100%	0.00%	0.00%	Lightning does 50/40/25 damage on direct hit, 10/8/5 if you touch the surface where it hit.
	Baristute	58/46/29	200	9.77%	36.13%	44.34%	9.77%	0.00%	0.00%	Also called big sidehoppers
	Charlie	50/40/25	100	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	Lightning does 50/40/25 damage.
	Charge Worm	12/9/6	60	0.00%	0.00%	68.36%	0.00%	29.30%	2.34%
	Charge Worm Spike	4/3/2	1	0.00%	0.00%	48.83%	39.06%	9.77%	2.34%
	Desgeega	30/24/15	40	0.00%	0.00%	48.83%	39.06%	9.77%	2.34%	Also called sidehoppers
	Dragon	18/14/9	18	2.93%	3.91%	9.77%	9.77%	48.83%	24.80%
	Dragon Fireball	8/6/4	1	0.00%	0.00%	19.53%	9.77%	29.30%	41.41%
	Gadora	25/20/12	60	0.00%	0.00%	50.00%	50.00%	0.00%	0.00%	Projectile does 30/24/15 damage.
	Gamet	8/6/4	4	2.93%	2.93%	19.53%	14.65%	39.06%	20.90%
	Gamet	16/12/8	13	5.86%	6.25%	24.41%	39.06%	24.41%	0.00%
	Geega	8/6/4	4	0.00%	0.00%	29.30%	9.77%	48.33%	12.11%	These geegas appear ONLY in the long zipline room above Acid Worm.
	Geega	8/6/4	4	4.88%	9.77%	39.06%	43.95%	2.34%	0.00%
	Geruda	30/24/15	40	9.77%	27.34%	43.36%	19.53%	0.00%	0.00%
	Geruda	16/12/8	10	2.93%	9.77%	19.53%	0.00%	19.53%	48.24%
	Hive	14/11/7	64	0.00%	0.00%	0.00%	100%	0.00%	0.00%	Only drops health if frozen
	Holtz	40/32/20	30	4.88%	75.59%	19.53%	0.00%	0.00%	0.00%	Also called owls
	Imago Cocoon	30/24/15	0	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
	Imago Spore	20/16/10	1	0.00%	0.00%	39.06%	9.77%	0.00%	51.17%
	Imago	10/8/5	750	0.00%	0.00%	75.59%	24.41%	0.00%	0.00%
	Imago Spike	20/16/10	1	0.00%	0.00%	80.47%	0.00%	19.53%	0.00%
	Imago Worm	20/16/10	30	0.00%	0.00%	0.00%	58.59%	41.41%	0.00%
	Kraid	20/16/10	300	0.00%	0.00%	0.00%	50.00%	50.00%	0.00%
	Kraid's Claws	4/3/2	1	0.00%	0.00%	48.83%	19.53%	19.53%	12.11%
	Mecha Missile	80/64/40	4	1.95%	1.95%	4.88%	3.91%	9.77%	77.54%
	Mecha Ridley	150/120/75	360	0.00%	0.00%	0.00%	100%	0.00%	0.00%	Laser does 50/40/25 damage, fireball does 60/48/30 damage.
	Mella	6/4/3	3	0.49%	0.98%	7.81%	0.00%	19.53%	71.19%
	Mellow	5/4/3	3	0.00%	0.00%	0.00%	0.00%	0.00%	100%
	Metroid	1/0.25/0.125	100	0.00%	12.11%	48.83%	39.06%	0.00%	0.00%	Damage is per frame, and it is the same on Easy/Normal mode.
	Mother Brain	20/16/10	700	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	Laser does 50/40/25 damage.
	Multiviola	18/14/9	12	2.93%	4.88%	9.77%	4.88%	29.30%	48.24%
	Nova	12/9/6	4	2.93%	2.93%	9.77%	0.00%	19.53%	64.84%
	Nova	22/17/11	12	4.88%	5.86%	19.53%	4.88%	39.06%	25.78%
	Polyp	0	20	0.00%	0.00%	100%	0.00%	0.00%	0.00%
	Polyp Projectile	8/6/4	1	0.49%	0.49%	1.95%	0.49%	9.77%	86.82%
	Ridley	30/24/15	1000	0.00%	19.53%	31.64%	48.83%	0.00%	0.00%	Body damage.
	Ridley's Tail	50/40/25	1	0.00%	0.00%	0.00%	0.00%	0.00%	100%
	Ridley Fireball	15/12/7	1	0.00%	0.00%	0.00%	0.00%	0.00%	100%	Fireballs shot at the start of the fight
	Ridley Fireball	30/24/15	1	0.00%	3.32%	82.03%	9.77%	4.88%	0.00%
	Rinka	20/16/10	7	0.00%	9.77%	56.05%	34.18%	0.00%	0.00%
	Rinka	10/8/5	7	0.00%	0.98%	34.18%	4.88%	0.98%	58.98%
	Rinka	10/8/5	7	0.00%	0.98%	58.59%	4.88%	0.00%	35.55%	These rinkas look identical to the red ones but have different drop rates. They are found by Mother Brain.
	Rio	12/9/6	16	4.88%	4.88%	9.77%	9.77%	19.53%	51.17%
	Rio	12/9/6	20	4.88%	4.88%	24.41%	24.41%	4.88%	36.52%
	Ripper	10/8/5	20	3.91%	7.81%	39.06%	7.81%	4.88%	36.52%
	Ripper	20/16/10	40	3.91%	29.30%	29.30%	19.53%	4.88%	13.09%
	Ripper	2/1/1	1	0.00%	0.00%	0.00%	50.00%	50.00%	0.00%	Only appears in Imago Cocoon's room. Can only be killed by cocoon falling on it.
	Ripper II	30/24/15	30	4.88%	29.30%	36.52%	19.53%	9.77%	0.00%
	Sidehopper	20/16/10	30	0.00%	4.88%	9.77%	19.53%	9.77%	56.05%
	Skree	10/8/5	3	2.93%	2.93%	9.77%	4.88%	9.77%	69.73%
	Skree	20/16/10	8	9.77%	9.77%	14.65%	14.65%	29.30%	21.88%
	Skultera	30/24/15	20	0.00%	0.00%	50.00%	40.23%	9.77%	0.00%	Fish
	Space Pirate	100/80/50	30	0.00%	0.00%	0.00%	0.00%	0.00%	100%	Shot does 100/80/50 damage.
	Space Pirate	50/40/25	72	0.00%	0.00%	0.00%	0.00%	0.00%	100%	Shot does 100/80/50 damage. Only vulnerable to Plasma Beam, which doesn't travel through it i.e. uncharged full beam combo only does 6 damage.
	Squeept	16/12/8	8	4.88%	4.88%	90.23%	0.00%	0.00%	0.00%
	Turret	10/8/5	1	0.00%	0.00%	0.00%	0.00%	0.00%	100%
	Vine	6/4/3	3	0.00%	0.00%	19.53%	4.88%	29.30%	46.29%
	Vine	10/8/5	6	0.00%	0.00%	29.30%	4.88%	24.41%	41.41%
	Vine	15/12/7	12	0.00%	0.00%	39.06%	4.88%	19.53%	36.52%
	Vine	20/16/10	18	0.00%	0.00%	58.59%	9.77%	19.53%	12.11%
	Vine	25/20/12	24	0.00%	0.00%	100%	0.00%	0.00%	0.00%
	Viola	16/12/8	12	2.93%	4.88%	19.53%	1.95%	19.53%	51.17%
	Viola	30/24/15	20	4.88%	9.77%	19.53%	9.77%	29.30%	26.76%
	Waver	12/9/6	8	1.95%	1.95%	4.88%	3.91%	9.77%	77.54%
	Zeb	6/4/3	2	4.88%	7.81%	24.41%	7.81%	48.83%	6.25%
	Zebbo	10/8/5	6	2.93%	9.77%	19.53%	19.53%	39.06%	9.18%
	Zebbo	18/14/9	14	5.86%	14.65%	30.66%	39.06%	9.77%	0.00%
	Zebetite	0	140	0.00%	0.00%	0.00%	0.00%	0.00%	100%
	Zeela	20/16/10	12	1.95%	1.95%	7.81%	11.72%	1.95%	74.61%
	Zeela	10/8/5	6	1.95%	1.95%	7.81%	2.93%	11.72%	73.63%
	Zoomer	10/8/5	3	4.88%	4.88%	21.88%	1.95%	29.30%	37.11%
	Zoomer	8/6/4	8	4.88%	4.88%	9.77%	14.65%	2.93%	62.89%	Also called Geemers

Drop Rates

Adapted from Dragonfangs' guide on SRC: https://www.speedrun.com/mzm/guide/dir80

Practical stuff

The game does a couple of calculations based on the enemy's position, its index in RAM and multiple RNG values to get a number between 1 and 1024 (the calculation is explained in the "Technical Mumbo Jumbo" part if you're actually interested). Controlling this number in any meaningful way is not reasonable in real time, for all practical reasons it is truly random.

This number is then tested against the raw drop rates derived from the table above. To get the raw value, multiply the number in the table by 1024.

If it's bigger than 1024-(rate for power bombs) you get a power bomb drop, otherwise if it's bigger than 1024-(pb rate)-(super rate) you get a super and so on, continuing with Missiles, Large health and Small health. After your drop is decided however, it can transform into something else based on your current resources.

For health(big and small) it does the following checks:

If health is not full: drop normally otherwise, if missiles are not full: drop a missile if both health and missiles are full: drop normally anyway

For everything else, it does the following:

Is the resources in question not full: drop normally otherwise, if health is not full: drop a large health otherwise, as long as you have the item unlocked: drop normally anyway or finally, failing everything else: drop a small health

So when looking at the drop tables, you can (in cases that matter) add both health drop rates to missile drop rate if you're full on health. And if anything else is maxed out, add it to large health drop rate.

Technical Mumbo Jumbo

The game uses two RAM values for the RNG calculation: 0x3000C77 and 0x3000002. I will refer to them as "frame1" and "frame2" respectively in the formula. These change every frame but can also change by invoking random elements (like checking for drops).

The formula starts as follows (% denotes a modulo operation):

((frame1 + 1) % 256 + ((frame2 + 1)/16) + (enemy's index in the list of enemies) + (enemy x position) + (enemy y position)) % 32

the result will then be used as an index to fetch a number from this list, I'll refer to the fetched number as "listRNG": {13, 2, 6, 8, 7, 9, 14, 10, 2, 4, 14, 4, 12, 15, 13, 12, 11, 1, 3, 15, 0, 6, 7, 8, 11, 5, 0, 3, 5, 1, 9, 10}

the calculation continues:

((frame1 + 1) % 256 + (frame2 + 1) + listRNG¤256) % 1024

and the resulting number is compared against the drop rates.

Note that metroids, in order to give two drops, are technically two enemies, with the second one dying at the same time and in the same location as the first one. So the only thing different between the two drops is the enemy index.