Lizard Man: The lizard man seems to have lost its two claw attacks, now having but a single attack at 1d8 damage. If this is a weapon attack as the description implies, then I guess that Lizard Men are adapting to more civilized warfare, rather than using their bites.
Lycanthropes: Holmes has included all five types from OD&D: wereboars, werebears, werewolves, weretigers and wererats. Werebears and weretigers have had their attack routines altered. Whereas before each of them had a few attacks of smaller damage, now each gets one attack with enormous damage. The werebear dishes out more damage than just about everything else in the game. The origin area of some lycanthropes is mentioned, with werewolves being from ‘Europe’, weretigers from ‘India’, and weresharks from ‘Polynesia’. Alas, there are no wereshark stats provided.
Lycanthropes are still only affected by silver or magical weapons, but now it is spelled out that they can be damaged by normal weapons while in human form. It’s also made explicit that they are repelled by wolfsbane. And of course there are the obligatory alignment changes: wereboars changed from Chaotic to Chaotic Good, wererats from Chaotic to Lawful Evil, werebears from Lawful to Chaotic Good, weretigers from Chaotic to Chaotic Evil, and werewolves from Chaotic to Lawful Evil.
Manticore: The manticore’s alignment has changed from Chaotic to Lawful Evil. Their attack routine has also been simplified, with each claw and bite dealing 1d6 instead of the claws doing 1d3 and the bite doing 1d8.
Medusa: As in OD&D, but with the amusing note that ‘this monster is usually female’.
Minotaur: Their alignment has changed from Chaotic or Neutral to Lawful Evil. Otherwise, they are just like OD&D.
Mummies: The Armor Class of mummies has dropped from 3 to 5. Their alignment has changed from Chaotic to Lawful Evil. The mummy has also developed a greater resistance to magic weapons; in OD&D it was the weapon's magic bonus that was halved, but in Holmes the entire damage rolled is halved. There's also a whole new rule about characters being paralysed by their first sight of a mummy.
I don't know if I realised this before, but the mummy's rotting touch is utterly hardcore. That it causes you to heal at 1/10th the normal rate is bad enough, but the best you can hope for in the way of aid from clerical magic is that your healing rate will be halved from now on. Once a mummy touches you, that's it buddy – that rotting disease has you forever. Throw in the new rule about mummy paralysis and you've got one very deadly creature.
Contradiction alert: mummies are said to be vulnerable to fire, yet burning oil only does half damage to them. What gives, Holmes?
Nixies: The same as in OD&D, except that the effect of Dispel Magic on their charm spell is not given, probably because that spell is out of the range of PCs using the Basic Set. The bit about flaming swords holding their fish flunkies at bay is also omitted, for reasons less easy to fathom. Also, they now have a movement rate of 60 on land.
Ochre Jelly: The same as OD&D, with the added ability to eat through leather and cloth.
Ogre: Their Armor Class has changed from 5 to 6. Their alignment has changed from Neutral or Chaotic to Chaotic Evil.
Orc: Their Armor Class has changed from 6 to 7. Their alignment has changed from Neutral or Chaotic to Chaotic Evil. There is no longer a roll to see if Orcs in the wilderness lair in caves or a village, and the powerful monsters that live with the Orcs have been drastically scaled back – there's no longer a chance for high-level Fighters or Magic-Users, or Dragons. Orcs no longer have a chance to be guarding wagon trains full of gold, either. I suppose this is because Holmes D&D doesn't really deal with wilderness adventuring, being more focussed on the dungeon. But taken as a whole it looks to me like the orcs are falling on hard times.