Chimera: In OD&D they could have an alignment of Neutral or Chaotic, but in Holmes Basic they are now Chaotic Evil. There’s also a small change to their damage dice. In Supplement I, the goat head is said to inflict 1-4 points of damage with a horn. Here it inflicts 2-8, which makes me wonder if in Supplement I you were supposed to roll the 1d4 damage for each horn. It’s clarified that the dragon head will breathe fire 50% of the time in any combat round, and that it is limited to 3 times per day. The damage dice is also upped from 3d6 to 3d8.
Cockatrice: Cockatrices were given no alignment in OD&D, but here they are listed as Neutral. They also get a physical description for the first time, as a chicken with a serpent’s tail.
Later supplements gave the cockatrice abilities relating to the Astral and Ethereal Plane, but they aren’t printed here, probably because it’s not a factor for the low levels that the Basic Set deals with.
Displacer Beast: Displacer beasts were listed as Chaotic in Supplement I, but in Holmes Basic they are said to be Neutral with Evil tendencies. Some other details are also dropped, such as their enmity with Blink Dogs (although it did get mentioned in the Blink Dog entry). The biggest change comes with saving throws. In Supplement I they saved as 12th level Fighters, which gave them some very good defences. In Holmes they are simply said to have a +2 to saving throws, further research of which has led me to discover that there’s no mechanism in this rule set to figure out saving throws for the toughest monsters. Unless I’m missing something...?
Djinni: It is clarified that djinni-created steel lasts for but 1 turn, and that their illusions can include sound. Their carrying ability is also different, with bigger loads tiring them more quickly. It’s clarified that creatures under 2 hit dice are killed outright if caught in their whirlwind form, and that stronger creatures still take 2-12 damage. They also now have an alignment of Neutral.
Doppleganger: Their alignment was listed as Neutral or Chaotic in Supplement I, and here they are either Chaotic Evil or Neutral; that seems much the same to me. Their ability to change shape is now limited to humanoids up to 7 feet tall.
Dragons: Only four types of dragon are given in Holmes: White, Black, Red and Brass. I suppose Holmes wanted to include the classic fire-breathing variety, then rounded things out by including the three weakest types. All dragons now deal 4-24 damage with a bite, instead of the variation by type presented in Supplement I. The alignment of Dragons from OD&D was either Neutral or Chaotic. White and Black dragons can still be Neutral or Chaotic Evil, while all Red Dragons are now Chaotic Evil. Brass Dragons in Supplement I were Lawful or Neutral, but here they can be Neutral or Chaotic Good – and if that switch from Law to Chaos isn’t an indicator that alignment is now serving a completely different function, then I don’t know what is. Their chance to use their breath weapon in any given round has slightly decreased. Cone breath weapons now have a smaller diameter at the dragon’s mouth. Dragon’s now have a larger chance to be small or large, and for the first time small dragons are specifically female, and large ones are specifically male. And there are now two extra age categories as well – young adult and ancient – which has also resulted in the age ranges for some categories increasing. It’s interesting to note that now only sleeping dragons can be subdued; in OD&D, characters could elect to subdue a dragon at any time. The method used for determining subdual is also much simpler, but also makes it harder for the PCs to accomplish; I can get behind both of these things. A maximum duration of one month is also now given for subdual, which wasn’t present before.
There’s a ton of information from OD&D that didn’t make it into Holmes Basic. There are no numbers given for the chance to find a dragon asleep, even though it’s mentioned in the rules for subduing. Likewise, there is no chance given for dragons to be able to speak. No info is given on the preferred habitats of each dragon. None of the resistances to various energy attacks are here. There’s also nothing about encountering dragon families. In short, there's a whole lot of simplifying going on.