I'm already on record as being against the longer stat blocks in the 1e Monster Manual and later; I even get a little miffed at the stat blocks in Labyrinth Lord and Swords & Wizardry. My main beef is that I don't think common information should be included in a stat block; I prefer to only include information that is different from the norm, that makes a creature stand out, which is why I use the one-line stat block, like this:
# Appearing: 3d6 x 10; HD 1-3, AC 8, Move 18This is all I need for a basic animal. Hit Dice and Armor change very frequently from creature to creature, as does the number appearing, so these will always appear in my "stat blocks". Move is usually 12, which I can leave out, but might be quite different for animals, based on locomotion method. I use 1d6 as base damage, up to 1d6 per full 4 HD, and modify the base by any additions or subtractions to the the Hit Dice, so I rarely need to note damage; these rats, for example, would do 1-3 damage. The word(s) in parentheses are a quick guide to creature behavior and function; stuff like Frequency and Alignment get noted here if they are different than the norm ("common" and "neutral".) Any special abilities or behaviors would be noted in a couple short paragraphs, but for most animals, those two lines, name and stat block, are all I need.
Which leads into the ranty portion of this post: variants. If the only real difference between two variants of the same base animal is Hit Dice, why list them as two separate creatures? The reason, I fear, is mere padding, for early games, and tradition for later games like the retro-clones. Further, there's the added problem that monster designers feel the need to make fiddly little changes to monster stats when making, say, a Blink Rat, because they feel dirty about doing a whole write-up on a new animal that's really just another animal plus a new behavior or ability.
I see no need for it. A simple note in the descriptive paragraph will do: "There are larger, rarer varieties with more hit dice." If a couple other things change, you might need an extra stat block line or two:
# Appearing: 3d6 x 10; HD 1-4, AC 8, Move 18
Enormous Rat (Rare Vermin): # Appearing 3d6; HD 1-2, AC 9, Move 15
But we can take this a step farther: for many animals, unlike monsters, there's not much difference other than size and behavior, so why even do a stat block line for them? A squirrel behaves differently than a rat, but is pretty close, so we don't need an entry for "Squirrel"; just use "Rat". We can, in fact, use the Rat stats for any rat-sized creature, and the Enormous Rat for any cat-sized creature, with perhaps a +1 or +2 to hit dice and a reduction in number appearing for actual predators. Change the Move to 3/Fly 60 and we have pigeon stats; change it to 18/Fly 36 and we have Winged Rats.
The real mechanical differences come in with special abilities. "Poison" is an obvious one, but that's as easy to handle: add a save vs. Poison for any victim bitten by a Poisonous Rat (or other creature;) failure means extra damage, or death for rare spiders or snakes; monstrous poisonous creatures always have deadly poison. There are a couple other special abilities, like the mauling attack of bears and wild cats, constricting attacks, binding with webs, throwing quills like darts, never check morale when defending young, keep fighting a few rounds after "death"...
What I think should have been done and is still desperately needed is an alphabetized list of such abilities with their mechanics and example animals, so that you can quickly modify basic animals to create a huge variety of species without the usual wasteful method of creating a full monster write-up and stat block for each one. For example, here's the maul attack, based on what I wrote about bears a while ago:
Maul Attack (bear hug, claw rake): damage roll of 6 on any die means automatic damage each round, no attack roll needed, until victim escapes or animal dies.
Constricting attack (for boas) would be the same, but would actually prevent movement by the victim, whereas the maul attack might allow a limited ability for self-rescue or defense. The important part is that we can now add this to other animals without needing a full write-up, so a Maul Rat is just "Rat, Enormous" + Maul Attack. It's almost like the template approach, but much simpler and less prone to number twiddling.
I'll probably have more on this later, like maybe a list of some animal abilities.