The Judges Guild Ready Reference Sheets are, as far as I know, the first D&D product to talk about monster intelligence in a systematic way. That is, books like Monsters & Treasure or Greyhawk may mention how intelligent a monster is in the description, but there is no attempt to do this for most monsters, and there is no translation of this into actual game rules; it's just in the description to enable the GM to role-play the creature.
The reference sheets have an actual table that sorts monsters into Unintelligent, Semi-Intelligent, and Intelligent, with a numeric range for each and a note on a few Intelligent monsters to indicate they are actually very intelligent. I'm of two mind about the Int scores: I'm OK with giving numeric equivalents for the purpose of judging some situations, but I emphasize in the games I run that Int scores are an abstract comparison. An Int 3 character is not a moron, incapable of speech or reason, unless a player chooses to play the character that way. So, I prefer using the same 3-18 scale for all creatures, but with Unintelligent, Semi-Intelligent, or Intelligent as a descriptive modifier.
However, on the good side, the JG rules only seem to apply to two situations: disbelieving illusions, and determining monster behavior. For all intents and purposes, an unintelligent creature is just a creature that always attacks when threatened or hungry, while other creatures will try to capture inferior opponents and intelligent creatures will try to avoid superior forces. This may explain some oddities in the table: bugbears are listed as Unintelligent, like a purple worm, and a large grey ooze is listed as Semi-Intelligent, like all the other humanoids. Hydras are listed as Intelligent; is that because they are capable of speech and rational thought, or only because they can judge the threat level of opponents and avoid the fights they are likely to lose? Do grey oozes occasionally capture opponents, to store in a larder?