It's never entirely spelled out in classic D&D exactly which creatures have their own racial language. OD&D says (Vol-1, p. 12), "The 'common tongue' spoken throughout the 'continent' is known by most humans. All other creatures and monsters which can speak have their own language..." But which ones are those?When my players have the opportunity to learn a new language, I'm in the habit of handing them the list of aligned creatures from that same book (Vol-1, p. 9), but the more I've used it, the more some of those types look suspect to me:Now, the AD&D game has more clues. The DMG (p. 102) has a random list of languages, so we can firmly settle for that game edition that all these creatures speak their own language:Note the last category (86-00) and its associated footnote; "other" can be any "unlisted creature language", so apparently appearance in this table is only sufficient, but not necessary, for a race to have its own language.We know that Intelligence is related to one's ability to speak a language (that's the ability that sets the number of languages one can know in every edition). Let's say we scan though the AD&D Monster Manual and see if we can correlate creature Intelligence to speaking a language.We find this: There are a half-dozen monsters who have their own language (per the DMG table above), and who have an Intelligence in the category of "Low" (meaning 5-7 points per the MM preface). These are: Ettins, Giants (Hill), Manticores, Minotaurs, Ogres, and Trolls. No creature in that table has an Intelligence any lower than that,; the Chimera is listed as Semi-Intelligent (2-4 points), and of that monstrosity it is written, "Chimerae speak a very limited form of red dragon language" (MM p. 14).So we might draw the pattern that creatures of Low Intelligence are most likely to speak their own language, whereas creatures with Intelligence below that usually don't speak, or at best, use a primitive pidgin version of some other race's. Based on that, it seems a fair bet that these other races also have their own languages in either edition, based on listed Intelligence in the MM: Treants (Very intelligent), Unicorns (Average), Pegasi (Average), and Wyverns (Low).Now, here are some types that would seem to have lower Intelligence in AD&D than would permit language use: Rocs (Animal), Hippogriffs (Semi-), and Gorgons (Animal). However, note that AD&D has definitely made a big change for each of those in at least one other aspect; whereas in OD&D they are all aligned (Rocs* and Hippogriffs being Lawful; Gorgons being Chaotic), in AD&D they are instead listed as Neutral. To me, this seems to mark a change in those creatures who were formerly seen as being sentient (some minimal intelligence required to be non-neutral?). So back in OD&D I would read their being aligned as implying intelligence, and therefore also language-speaking. Likewise (since Pegasi and Hippogriffs both presumably speak), I would take the Griffon as also being also able to do so, even though it is Neutral in both editions.To some extent OD&D echoes in this way the fantastic freedom inthat almost any creature seems to possess intelligence and some unique language, whereas AD&D andseemed to get more restrictive/mundane in this regard.This leaves a very small number of monsters from the OD&D list as possibly not speaking any language, creatures that are Neutral in every edition, and also have very low Intelligence in the Monster Manual: Hydras (Semi-Intelligent), Purple Worms (Non-Intelligent), and Sea Serpents (as a Plesiosaurus per Arneson in Sup-II; Non-Intelligent in the MM). Now, Hydras have the same Intelligence as Chimeras, so I'm prone to say they can speak, perhaps again in a degraded form of Dragon or whatever. The entirely Non-Intelligent types (which means 0 points per the MM), the Purple Worm and Sea Serpents, I'm pretty comfortable as leaving non-speaking mindless monsters. But that's (surprisingly) only two out of the entire OD&D list!I guess (having mentioned), I should address the "Animals" entry in that same OD&D list. Certainly I wouldn't want a single language for all "Animals" (as some new players of mine have hoped for, given the way I introduced that table). Per, maybe every animal type has its own language. But following the Monster Manual, I think I'd require the type to have above "Animal Intelligence"; I'd be comfortable with any giant type so speaking?* Finally: Note the particularly interesting evolution of the Roc in D&D. In the earliest printing of Chainmail, it is expressly written that, "These equal the 'Eagles' of Tolkien's Trilogy". And again in the first printing of OD&D (Vol-2): "This term has been used to encompass large and fierce birds such as the 'Eagles' of Tolkein", as well as expansion on their strong sense of alignment: "they will be positively hostile only to Chaos and Neutrality, ignoring (80%) or being friendly (20%) to Lawful characters".In contradistinction, AD&D seems to split these types apart, with a separate entry for Giant Eagles (MM p. 36) with Average Intelligence and who "have their own language", versus Rocs (MM p. 82) who are now switched to Neutral and given a lowly Animal intelligence (and hence presumably non-speaking in that edition, by my analysis). In OD&D I'm happy to keep the original intent as sentient Tolkienian Eagles intact (and also not multiply entities needlessly).