|A Black Pudding has no alignment, but a Carrion Crawler is neutral|
The alignment scheme in Holmes is generally referred to as "five-point". This scheme is adapted from the Strategic Review #6 (Feb '76) and includes lawful good, chaotic good, neutral, lawful evil and chaotic evil. These are all that is described in the section on "Character Alignment" (pg 8). But there's actually a sixth option for monsters: no alignment. A number of monsters simply lack a line for Alignment. The 1st edition contained a nonsensical explanation for this, presumably due to an editing error ("If the monsters' alignment is given here, then there follows a brief description which should include any special powers and attributes of the creature"). This was corrected in the 2nd edition (Nov '78): "If the monster's alignment is not given, it may be assumed to be an unintelligent beast that will attack anyone who comes near".
The following monsters in the 1st edition of Holmes have no entry for alignment:
Black Pudding, Gelatinous Cube, Giant Tick, Gray Ooze, Green Slime, Horse, Ochre Jelly, Yellow Mold
The 2nd/3rd edition adds the following to the above:
Fire Beetle, Giant Ant, Giant Centipede, Giant Rats, Shrieker, Spider*
*Giant Spiders are described as chaotic evil in the entry, being of low intelligence.
As with other products from this era, usage is not always consistent. The entry for Purple Worm says they are "unintelligent and always attack", exactly the conditions for having no alignment, yet they are neutral. Basilisks and Cockatrices are also described as unintelligent but are neutral. Skeletons (and Zombies) are listed as neutral, although they seemingly have no intelligence of their own and "act only under instructions of their motivator, an evil magic-user or cleric".
In terms of game play, the sixth option provides an economical way of indicating whether a reaction roll should ever be made for that monster. "Unintelligent" monsters are also the only ones possibly deterred by food (pg 11) and can never accept a surrender (pg 21). The Monster Manual went in a different, more standardized direction, including both an alignment and an intelligence for every monster.
Another implication of this scheme is that some "neutral" monsters could be more intelligent than normally assumed based on other rule sets. Carrion Crawlers, Hydras, Owlbears, Rust Monsters and Stirges are all "neutral", and thus do not necessarily attack on sight. By way of comparison, they could be considered more intelligent than other creatures lacking alignment such as Giant Rats and Horses.
Up for a parley with that Carrion Crawler coming towards you on the ceiling?