In retrospect, there were many telltale signs that Octillery were smarter than humanity realized. They learn psychic attacks and pick up rocks – tools, weapons - in order to hunt their prey, even in the so-called wild. With a human trainer, they are remarkably adaptable, and can learn even fire attacks with remarkable ease. The oddest thing about Professor Elm's recent discovery was not that the Octillery ink marks so often found in underwater caves were a form of writing, it was that it took so long for anyone (save a few presumed crackpots) to notice.
Human language is based to a large degree on the powers of the Unown and the vocalizations of countless wild pokemon, yet this has proven little use in deciphering the Octillery script, a strange array of ink which resembles no letters or symbols known to man. What has been learned from it is little; no one knows if they tell stories on cave walls and inside their burrows, keep records, or merely relay elaborate orders in astonishing detail. But it is now known that Octillery ink is not a naturally occurring substance, but one carefully crafted in small holes from undersea minerals, eaten, and spit out by Octillery to produce messages which other Octillery clearly comprehend. The rest is conjecture.
Interestingly, when the black ink ball called Octazooka explodes on an Octillery's opponent, many of the signs seen in its explosion bear a striking resemblance to those commonly seen in undersea caves. Perhaps this is a coincidence, but most consider it likely that Octillery write messages on their target when they attack which only their own kind can understand.