Mammoth: Mammoths first appeared in the wilderness encounter tables in OD&D Vol. 2, but they get stats here for the first time. They’re pretty much exactly the same as elephants, only with more Hit Dice, and the ability to dish out a lot more damage. Their tusks are worth 1½ times as much as an elephant’s, as well. There are two types of mammoth, woolly and imperial, and I’m shocked that Gary didn’t separate them and give each its own set of stats.
Manticore: Manticores first appeared in OD&D Vol. 2. Gary must have really, really liked the way he designed this monster, because the only change he’s made is to give it two extra hit points. (Although now that I think of it, those two extra hit points, taking it from 6+1 to 6+3 Hit Dice, mean that it attacks as a 7 Hit Dice creature, doesn’t it?)
Masher: This monster first appeared in Supplement II. It’s still a gigantic worm-like fish, but statistically it might as well be an entirely new monster. The old version was pretty much exactly like a purple worm, with a poisonous tail and the ability to swallow prey whole. The new masher has neither of these abilities, but it does have poisonous spines that can impale anyone who tries to attack it.
Mastodon: Mastodons first appeared in the wilderness encounter tables in OD&D Vol. 2, but they get stats here for the first time. It’s kind of ironic that I was surprised earlier about Gary not making stats for both kinds of mammoth, because the mammoth and the mastodon are almost EXACTLY THE DAMN SAME. I give Gary points for thoroughness, but there's a lot of needless redundancy in the first Monster Manual.
Medusa: This monster first appeared in OD&D Vol. 2. It’s been beefed up a little since then. Its Armor Class has improved from 8 to 5, and it has 6 Hit Dice instead of 4. I suppose Gary thought that it was a pretty weak monster with such a bad AC, and I’m inclined to agree with him; even with their eyes shut, PCs aren’t going to have much trouble hitting one. Their petrification gaze attack has now been given a range, but the biggest change here is cosmetic. In OD&D, medusas had the lower body of a snake. Now they are fully humanoid, with legs and all. They're now much more in line with their portrayal in pop culture, but it presents me with a problem. We have weaker mudusas with snake bodies in OD&D, regular medusas with legs in AD&D, and the Greater Medusa in AD&D 2e that has a snake body. Two separate species, perhaps? The two types of snake-bodied medusas could just be younger and older varieties of the same thing. The humanoid variety have to be different somehow. Maybe they're the result of a snake-bodied medusa mating with a human? (Just forget the logistics of the union and roll with it...)