Giant Animals. We're all familiar with these. Things listed as "Giant X" where X is some normal animal type include: Rats, Hogs, Ants, Snakes, Weasels, Beetles, and Scorpions. Most of these are in monster levels 3 or 4 (although Giant Rats are level 1). All of these have some analogous entry in the AD&D Monster Manual, so post-1977 you could use that resource to fill things in (recognizing that some monsters got boosted in the OD&D --> AD&D switch; see the last post). But what would the DM do running with just OD&D itself? Some guidelines in Vol-2:
LARGE INSECTS OR ANIMALS: This category includes giant ants and prehistoric monsters. Armor Class can be anything from 8 to 2. Hit Dice should range from 2 to anywhere near 20, let us say, for a Tyrannasaurus Rex. Also included in this group are the optionally usable "Martian" animals such as Apts, Banths, Thoats, etc. If the referee is not personally familiar with the various monsters included in this category the participants of the campaign can be polled to decide all characteristics. Damage caused by hits should range between 2-4 dice (2-24 points). [OD&D Vol-2, p. 20]A few comments here: Taken literally, the rule in the text above (min 2HD) would make for really heavyweight Giant Rats, which are of course level 1 and given only 1/2 hit die in AD&D. Note also that the damage parameters are rather terrifying -- in OD&D, 2 dice is the same as a Hill Giant, while 3-4 dice is the same as a double- or triple-size Purple Worm (i.e., Sea Monster). Personally, looking at the Wandering Monster charts, my first guess would have been to assign a number of Hit Dice according to whatever monster level is indicated -- and you'd be accurate to the AD&D Hit Dice (or one less) in each case.
Normal Animals. Oddly, there are a few animals pointedly listed without the "Giant" identifier. These are: Centipedes, Spiders (level 1 each), and Lizards (level 2). What to make of these? Obviously, in AD&D they're each listed as a Giant type with attacks like weak poison, but there's no indication of that in OD&D. They seem to be referenced in this entry from the monsters book:
INSECTS OR SMALL ANIMALS: These can be any of a huge variety of creatures such as wolves, centipedes, snakes and spiders. Any hit will kill the smaller, while larger beasts (such as wolves) will receive one Hit Die. Generally speaking they will be Armor Class 8. [OD&D Vol-2, p. 20]So perhaps at one point the Centipedes and Spiders listed in OD&D effectively had just 1hp? Were we meant to intuit a poison attack, if anything? It's hard to tell here. (One other thing that I point out elsewhere is how ridiculously huge the 3HD Giant Lizard is in AD&D -- pretty much like every monster given a specific size in feet there -- 15' long, whereas all other monsters in OD&D from levels 1-4 are basically human-scaled.)
Thouls. So, Thouls are listed in the OD&D monster level 2 chart, but they were never given statistics in any OD&D or AD&D publication. Looking only there, I might think to give them 2HD (by level) and some abilities like Ghouls (of whom they seem cognate, and they come immediately prior to in the encounter table). Of course, Moldvay Basic D&D later gave them 3HD, paralysis, regeneration like a troll, and the appearance of a hobgoblin. Perhaps more interestingly, we have evidence that the original Greyhawk Castle notes show the monster with 4HD and similar abilities -- which would make it distressingly powerful compared to other creatures on the level 2 list. (This information from Gygax, via Gene Weigel, via T. Foster over at the Dragonsfoot boards.)
White Apes. These occur in the level 4 listing. Granted all the "John Carter from Mars" references, it's pretty obvious that these refer to the White Apes of Barsoom, who appear alongside such types as Apts, Banths, etc. (mentioned in the first quote above). These are huge, 4-armed, semi-intelligent gorillas lacking any direct statistics in D&D. I assume that AD&D's "Ape, Carnivorous" (5HD) is the replacement here, although it only has standard limbs. 3E D&D has the "Girallon" monster which is quite obviously the White Ape under different trade dress (7HD, huge, white-furred, and 4-armed; stats here and picture here).
Other Considerations. The preceding account for all of the OD&D Wandering Monsters that lack statistics in that game. Here's a few other thoughts:
Let's say you think to replace the missing monster stats with stuff from the AD&D Monster Manual. You've got to be careful for a few reasons. First, note that some animals may not have been intended as "giant", but that's all you have available in the MM. Second (and related), note that some amount of Hit Die inflation occurred between OD&D and AD&D, and that might be true for the missing monsters as they appear in the MM. Third, you've got to make some decisions about exactly which AD&D monster of several sub-types should fill the slot (my suggestions, generally looking for appropriate Hit Dice and size: Spiders-->Large Spiders, Lizards-->Giant Lizard, Giant Hogs-->Wild Boar, Giant Beetles-->Boring Beetles, Giant Snakes-->Poisonous or Constrictor Snakes?) Fourth, and again related, some of these types will skew more dangerous than others on the same list (e.g., poisonous large spiders HD1+1 at level 1, giant lizards HD3+1 at level 2). In particular, using either of AD&D's Poisonous or Constrictor Giant Snakes at level 3 is really dangerous (respectively HD 4+2 or 6+1 by special attacks each; compare to Ogres listed at level 4 without any instant-death ability) -- I recommend a 3HD Constrictor type at that level.
One final peculiarity is that, for some reason, most of these types have damage entries listed in Sup-I Greyhawk, but are still missing the more basic stats (AC, MV, HD, etc.) To see this, remember that throughout OD&D, basic monster stats were in a separate table/location from attacks and damage (those being introduced in Sup-I -- and really this whole issue is what made the Monster Manual product so highly desirable and the first publication in the AD&D line). So if you look at the new, large Sup-I attacks/damage table, although they're all strung one-after-the-other in a single unified listing, you see three identifiable sections: 1st, all the original monsters from Vol-2 (Sup-I p. 16-18); 2nd, "missing monsters" of the type we discuss here (midway down p. 18); and 3rd, all the monsters being introduced elsewhere in Sup-I (p. 18-19).
Just looking for a moment at these orphaned damage entries, we see these types -- Giant (Sumatran) Rat, Wolf, Dire Wolf, Lion, Sabre-tooth Tiger, Giant Weasel, Mastodon, Giant Spider, Giant Lizard, Giant Toad, Giant Snake, Giant Crab, Giant Beetle, Giant Scorpion, Crocodile, Tyrannosaurus Rex. Note that the spider and lizard are now explicitly "Giant". While many of these damage stats match what you see in the Monster Manual, that's not the case for any of the monsters with multiple sub-types in AD&D, so you can't use that to synchronize them (e.g., snakes, spiders, beetles). For example, the entry for the Giant Snake indicates Attacks: 1 bite/1 constriction, Damage: 1-6 bite (poison noted), and 2-8/turn of constricting; now, if I allege above that the poisonous or constrictor snakes from AD&D are particularly scary, imagine an OD&D snake of the same hit dice that's both poisonous AND constricting. Holy smoke! (Recommend bumping that to a higher monster level if you use it.)
But irrespective of these Sup-I damage listings, there are still no basic stats anywhere in OD&D for our "missing monsters". Were they originally intended to be in Sup-I Greyhawk and got edited out at some point? Interesting to think about.