Giant Porcupine: Giant porcupines made their first appearance in the Wilderness Encounter Tables from Supplement III: Eldritch Wizardry. This is the first time they get stats. They’re quite tough, having 6 Hit Dice, and despite being described as non-aggressive unless threatened they consider any approach within 30 feet to be a threat. Which makes the likelihood of a battle pretty high for all but the most cautious of PCs. Their main attack form is to shoot quills from their tail, which does seem over and above the usual qualities of a porcupine. They also have defensive quills, and any attacker within a certain range will have to deal with being impaled. None of this is particularly interesting, but I think there would be a certain shame in having a character killed by a giant porcupine, and a certain pride in being the DM to inflict such a fate.
Portuguese Man-O-War: The Portuguese man-o-war (a type of jellyfish) first appeared in Supplement II: Blackmoor, but it has been given a total overhaul here. Number Appearing has changed from 2-12 to 1-10; Armor Class has worsened from 8 to 9; and Hit Dice was 2, but now ranges from 1-4. Originally, the creature was said to have 1-100 attacks a round, each with a paralysation effect, which is plainly unworkable. Here their number of tentacles depends upon size, and ranges from 10 to 40. Now they only get one attack a round, still with paralysation. As in the earlier version, their tentacles can be severed with a single point of damage, and they can only be killed by damage to their central body. Their transparency has also been given a concrete game effect, making them 90% undetectable. So the principle behind the monster is the same as before, it has just been greatly clarified to become something that can actually work in the game.
Pseudo-Dragon: This is the first appearance of pseudo-dragons. A pseudo-dragon is a small telepathic dragon-like creature with a chameleon-like power, and a poisonous stinger. The poison makes its victim appear dead for a few days. 1-in-4 victims actually die, but I can imagine that a lot of survivors are still consigned to an awful fate by their fellow PCs. They also have a decent magic resistance, and this is their most interesting feature, because they can confer it upon any creature they are touching. It is mentioned that they may become the companion of a humanoid, and I expect that this is covered further in the Players Handbook or the Dungeon Masters Guide. It would certainly be a most sought-after power for any PC.
Purple Worm: Purple Worms first showed up in D&D Vol. 2. They’re a little faster than they were before, with a Movement increase from 6 to 9. The damage from their stinger has changed slightly from 1-8 to 2-8. Otherwise it is the same, with a poisonous stinger and the ability to swallow opponents whole. What has been added is mostly clarifications, such as a note that the worm only uses its stinger in rear defense, or against large opponents in a spacious area. Rules are given for how large a creature it can swallow, and also for characters who want to cut their way out of the worm’s stomach. There are even stats for hatchlings. And apparently purple worms expel such indigestible waste as “metal and mineral crystals”. Gary doesn’t come out and say it, but that sounds like gold and jewels to me. Even the aquatic mottled worm from Supplement II gets a mention.