AD&D Monster Manual part 53

GIANT TICK: Giant ticks first appeared in Supplement I: Greyhawk.  They've changed very little, with just a few minor stat changes and clarifications to their abilities.  Their major attack is to attach to their victims and drain their blood.  Originally the tick drained 4 hit points per round, but now it will drain from 1-6, and become sated when the amount drained equals its total hit points; in the earlier version the tick would simply drain blood until the victim was dead.  Ticks can still be removed by killing or burning them, but a new method has been added here: immersing them in water.  Giant ticks still pass on a fatal disease to anyone whose blood they drink, but whereas before the disease was automatically passed on, now there is only a 50% chance.  Supplement III: Eldritch Wizardry established that the disease ticks pass on is called spotted fever, and gave some extra rules regarding it.  None of these additions appear here in the Monster Manual.

Stat Changes:
Armor Class: Old - 4, New - 3; Hit Dice: Old - 3, New - 2 to 4

TIGER: This is the first proper stat-block for tigers, as they've only appeared so far in the wilderness encounter tables from Supplement III: Eldritch Wizardry.  They're surprisingly strong, with 5 Hit Dice and three attacks per round.  In addition they're really hard to surprise, and if both of their claws hit in a single round they get two more attacks with their rear claws.

SABRE-TOOTH TIGER: Sabre-tooth tigers were first shown in the random encounter tables in the OD&D boxed set, and they also got attack and damage scores in Supplement I: Greyhawk, despite never having been properly outlined.  They pretty much function as a stronger version of the tiger, having the same abilities with more hit points, and an attack bonus due to their over-sized teeth.

Stat Changes:
Damage: Old - claws 1-4 and bite 1-12, New - claws 2-5 and bite 1-12 

TITAN:  The existence of titans was rumoured in the OD&D boxed set, and they got their first stats in Supplement I: Greyhawk.  They were powered up in Supplement III: Eldritch Wizardry, gaining psionic powers, magic resistance and a tendency to pal around with storm giants.  They retain all of their considerable abilities here, including psionics, magic resistance, and the ability to cast cleric and magic-user spells of up to 7th level.  In addition, they gain the following abilities: invisibility, levitation and etherealness, all at will.

Originally, titans were able to cast protection from magic at double strength.  Except for one problem: there is no such spell.  That's been amended here to protection from evil, and it's only double strength against Lawful Evil creatures.

I'm somewhat disappointed to note that the bit about there only being ten titans in total has not made it into the Monster Manual. We;re also back to referencing real-world places, with the garb of titans being described as "Grecian".

Stat Changes:
This  one is a little difficult to figure out, because of the way titans are set up.  Originally they had an Armor Class ranging from 2 to -3, hit points ranging from 75 to 100, and a Movement that was usually 15", except for a 10% minority that moved at 21".  It's been codified a lot better in the Monster Manual, with a roll of 1d6 determining the AC and HD of the Titan.  AC still has the same range, but now Titans have between 17 and 22 Hit Dice.  Those with smaller Hit Dice have the higher speed, and the stronger ones have a higher damage range.

AD&D Monster Manual part 52

SU-MONSTER: These creepy psychic monkeys first appeared in Supplement III: Eldritch Wizardry.  They've changed little, even using much of the same wording from the original entry.  As before they have a latent psychic ability, and can lash out with a randomly determined psychic attack is psionics are used within a certain range of them (psychic crush, psionic blast or mind thrust).  They also retain the same likelihood to appear as a family unit, with a father, mother and children.  If the children are attacked the mother fights at double value, and if the mother is attacked the father does the same.

Su-monsters were originally described as being "highly evil", but here their alignment is given simply as Chaotic.  This is odd for the Monster Manual, as most of the alignments do have the good-evil axis specified.  I think I'll fudge this one, and continue to read them as Chaotic Evil.  The only other change is that they're bit tougher, with an extra Hit Die and higher damage on their attacks.  Allow me to continue using the same explanation I've been leaning on too much lately: "these are the full-grown adults, and the older ones were not quite fully mature".

Stat Changes:
Hit Dice: Old - 4+2, New - 5+5; Damage: Old: 1-3 per claw & 1-8 per bite, New - 1-4 per claw & 2-8 per bite

SYLPH: Sylphs, appearing here for the first time in D&D, are a sort of aerial nymph.  They can cast spells as though they are 7th-level, although it's not specified whether they cast as clerics or magic-users.  Cleric spells seem to me to be more fitting.  They can also turn invisible at will, and conjure an air elemental once per week.  The one bit of personality they are given is that they have a 20% chance to befriend a good-aligned character and help them out in some way.

THOUGHT-EATER:  Brrrrr.  Thought-eaters give me the willies.  They appear here for the first time.  Thought-eaters dwell in the Ethereal Plane, but they're able to sense any psionic energy used nearby.  Once a thought-eater is close it will consume any spell or psionic ability that its victim uses; it will be sated once it has eaten from 101-200 points.  If the thought-eater gets even closer it can begin feeding on the victim's Intelligence score, resulting in permanent stat loss.  The rate of such feeding isn't specified, but I'd be inclined to make it 1 point per round, or perhaps an even slower rate.  They can only be killed by other ethereal characters, but spells such as mind blank and other defenses against psionics will be effective at keeping them away.

AD&D Monster Manual part 51

STAG and GIANT STAG:  Stags have only appeared previously in the Wilderness Encounter tables from Supplement III: Eldritch Wizardry, and giant stags have not appeared at all.  There's honestly not a lot to say about either of them.  Regular stags wander around in herds which they protect aggressively.  Giant stags do the same thing, only giant-er.  I'm all in favour of having regular animals in the Monster Manual, but they don't always make for the most compelling entries.

STIRGE: Stirges first appeared in Supplement I: Greyhawk.  These weird bird-mosquito hybrids have long been the scourge of low-level adventurers everywhere: a 1 hit-die monster that hits like it has 4 hit dice, and attaches to its victims to suck their blood.  They appear in large numbers as well, just to add to their deadliness.

In general they're another case of Gary being pretty happy with his previous work, but there are a few minor statistical changes as noted below.  The most significant of these is the addition of a speed for when they are grounded, and also a limit to how much blood they can drink.  Previously a stirge would keep drinking until its prey was a bloodless corpse; now it will drink until it has drained 12 hit points, before flying away to digest.  The only notable omission is a method for detaching a stirge from its victim while it is drinking their blood.

There isn't too much for me to explain away here.  The new stirges are slightly bigger (thus explaining both the extra hit point and the slightly worse Armor Class), but they don't drink quite as ravenously.  Again, I will go back to my old stand-by excuse: the previous version of the stirge was not quite fully grown, and still in need of lots of blood in order to fully mature.  The Monster Manual version is fully mature, and doesn't require quite the same amount of sustenance.

Stat Changes:
Armor Class: Old - 7, New - 8; Movement: Old - 18", New - 3"/18"; Hit Dice: Old - 1, New - 1+1

Brother, you been STIRGED.

STRANGLE WEED: Strangle weed was first detailed in Supplement II: Blackmoor, but here it gets a complete overhaul.  Conceptually it hasn't change a bit; it's still seaweed that grabs and constricts anything that gets too near.  However, the Supplement II version of strangle weed had a method of resolving that constriction that didn't take the victim's Strength into account at all.  In the Monster Manual it's been changed completely.  Each frond of the weed gets its own Strength score; if the victim's Strength is higher he gets a chance to escape, and if it is lower he takes crushing damage.  He can still try to hack his way to freedom, but with a -2 penalty to attack.  Multiple fronds can grasp a character, and their Strength scores are added together.

The original strangle weed had an Armor Class of 1 and 12 Hit Dice, which is actually pretty tough.  The new version has Armor Class 6 and 2 to 4 Hit Dice (presumably for each frond).  It's a major difference in power, and I'm willing to chalk these up as two different yet similar species.  The original version is obviously made of much sterner stuff, and possibly injects its victims with a poison that negates their strength.