This hiatus has lasted long enough. Let's do this!
Shambling Mounds first appeared way back in The Strategic Review #3, and have otherwise only been mentioned in the absurdly comprehensive Wandering Monster tables for Supplement III: Eldritch Wizardry. As with most of the creatures created by Gary for The Strategic Review, Shambling Mounds have been changed very little (even down to the exact wording of certain sentences). What we have is a living mass of vegetation that has a whole load of resistances and the ability to suffocate any character that it hits twice in any melee round. The deadliness of the suffocation ability has been softened somewhat; instead of a character dying in 2-5 rounds, it now takes 2-8. Balancing that out is a serious omission. In OD&D, the Shambling Mound could not attack while it was suffocating someone. In AD&D, that restriction isn't mentioned, so while characters might take longer to suffocate, Shambling Mounds have probably become deadlier on the whole.
Shamblers' immunity to fire, resistance to cold, and growth when struck by lightning remain unchanged, and they still take half-damage from all weapons. The only difference made is that their vulnerability to the spells plant control and charm plant has now been broadened to encompass other spells that affect plants.
Shambling Mounds' hit point determination has also been standardised. In OD&D they used d10s, but here they use d8s like every other monster. It evens out pretty well, because the number of Hit Dice has been upped. The strongest variety of Shambling Mound used to have from 9-90 hit points. Now it has from 11-88, a negligible difference. I'm sure the average totals are different, but no so much as to drastically change the monster's power level.
Hit Dice: Old - 6-9 (using d10s); New - 8-11 (using d8s)
Sharks were previously covered in Supplement II: Blackmoor, where they were described as Giant Sharks. In the Monster Manual we're given stats for regular Sharks and Giant Sharks, but the stats given in Supplement II more closely resemble those for the regular variety. I suppose that what was considered giant back then has been superseded with the discovery of Sharks of still greater size...
Sharks were described in very rudimentary terms in Supplement II, so what we get here is an expansion of their behaviours and abilities. They're given the ability to detect noise underwater up to a mile, and can scent blood from a similar distance. They're said to be particularly vulnerable to ramming attacks, though there is nothing mechanical to back this up. My personal favourite bit? Any Shark that is motionless will die in 2-5 melee rounds. It's not entirely scientifically accurate, but it does make fighting Sharks a bit different from other combats.
The damage ranges given for regular Sharks are a little ambiguous. Three separate ranges are given, but there's no indication of which to use. Presumably it's linked to Hit Dice, and there are conveniently six possible Hit Dice totals. It probably breaks down like this: sharks with 3-4 Hit Dice do the lowest damage, 5-6 use the middle range, and 7-8 use the highest range.
Giant Sharks come in two varieties: Giant White Sharks, and prehistoric Megalodons. They're functionally identical. Where they differ from regular Sharks (besides having a lot more hit points) is their ability to swallow creatures whole. If that happens, you've got six rounds to bring the Shark to 0 hit points, or that swallowed character will be digested. Attacks from within a Shark's stomach begin with a -1 penalty to damage, and that penalty gets bigger every round.
Stat Changes (comparison between Giant Sharks from Supplement II and regular Sharks):
Number Appearing: Old - 2-12; New - 3-12
Armor Class: Old - 5; New - 6
Hit Dice: Old - 4 to 9; New - 3 to 8