Analysis

So, I made a database of all the monsters in the 3.5 Monster Manual. Here are some interesting bits of information.

There are 614 monsters. This includes variations of the same monster (dragons of different ages, template examples, etc.) but does not include every possible variation (say from advancing monsters, applying classes, or using templates). The later would push that number into the thousands almost instantly.

The most common challenge rating is 3, by a wide margin. There are 68 monsters with a CR of 3, the next closest is 7 with 48 monsters. Curiously, the average CR is 7. Curiously, level 7 allows the greatest variety of encounters (assuming the -8 levels/+8 levels range from the DMG xp chart). Maybe it should be Epic 7 instead of Epic 6.

There are more monsters with the dragon type than any other. This is largely due to counting each age of a specific type of dragon. However, there are 125 monsters in the MM of the dragon type and the next closest is Outsiders with 96 monsters. Animals (81) round out the top three. Oozes, Fey, and Plants are the least common.

The most common climate/terrain is Temperate Forest (62 monsters) with Underground (46) and Temperate Plains (35) making the top three. The climate/terrain with the least variety (ignoring certain planes of existence) is Cold Deserts (just the Remorhaz)

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There are more Medium (179) and Large (178) monsters than all other sizes combined.

The most common alignment is Neutral (265). The next most common is Chaotic Evil (103) and Lawful Evil (69). Lawful Good (50) is the fourth most common, while Lawful Neutral (9), Chaotic Neutral (10), and Neutral Good (17) fill the bottom slots. If we are just talking Good vs. Evil (ignoring law, chaos, and neutrality), then Evil is more common (219 vs. 109) by double.

Other interesting tidbits.
The Nightshade Nightcrawler is the strongest monster (Strength 48)
The Elder Air Elemental is the quickest (Dexterity 33)
The Titan is the toughest (Constitution 39)
The Great Wyrm Gold Dragon is the most intelligent (Intelligence 32), the wisest (Wisdom 33), and the most charismatic (Charisma 32).

Actually, the Gold Great Wyrm is also the second strongest and third toughest...and has the highest Challenge Rating (for good reason).

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So what can we say about the world of D&D?

Well, it's highly predatory and dragons are the most common variety of life. Creatures of between CR 3 and 7 are mode and mean of the world. Those above seven are likely apex predators while those below 3 are likely preyed upon. Those between 3 and 7 represent part of an intense competition. The world is generally neutral, but you are more likely to run into an evil creature than a good one.