Page 134 of the Hurley translation of Imaginary Beings contains the following:
The Ink Monkey
This animal is common in the northern regions and is about four or five inches long; it is endowed with an unusual instinct; its eyes are like carnelian stones, and its hair is jet black, sleek and flexible, as soft as a pillow. It is very fond of eating thick China ink, and whenever people write, it sits with folded hands and crossed legs, waiting till the writing is finished, when it drinks up the remainder of the ink; which done, it squats down as before, and does not frisk about unnecessarily.
This entry is attributed to Wang Tai-hai in 1791. In the accompanying end note, Hurley refers us to a book called The Chinaman Abroad: An Account of the Malayan Archipelago, particularly of Java, written by Ong-Tae-Hae and translated by W.H. Medhurst in 1850.
That book exists, and can be accessed here. On pages 46-47 we find this:
The Ink Monkey
(46) Is common in the northern regions and is about four of five inches long; it is endowed with an unusual instinct; its (47) eyes are like carnelian stones, and its hair is jet black, sleek and flexible, as soft as a pillow. It is very fond of eating thick Chinese ink, and whenever people write, it sits with folded hands and crossed legs, waiting till the writing is finished, when it drinks up the remainder of the kink; which done, it squats down as before; and does not frisk about unnecessarily. Ong-te-hoe used to keep one at the head of his ink-stone, or in the middle of his seal-box.
The author hedges his accuracy a bit when, in his preface, he writes, "Although far from being intelligent, I dare not refuse carefully to record the things which I have seen and heard, together with some references to the country and its inhabitants, in shirt every individual word and action worthy of being noted down; thus publishing the whole, in order to render some small assistance toward correcting men's minds, and sustaining right principles in the world!"
Here's the other translation of the story, by di Giovanni:
The Monkey of the Inkpot
This animal, common in the north, is four of five inches long; its eyes are scarlet and its fur is jet black, silky, and soft as a pillow. It is marked by a curious instinct--the taste for India ink. When a person sits down to write, the monkey squats cross-legged near by with one forepaw folded over the other, waiting until the task is over. Then it drinks what is left of the ink, and afterwards sits back on its haunches, quiet and satisfied.
In an essay on Borges titled "Repetition, Museums, Libraries," Alicia Borinsky opens with Norman Thomas Di Giovanni's translation of the Ink Monkey story. Then uses that image, of the satisfied monkey, to get to this point: "The position of the monkey regarding the writer is the double nature of authorship; no text is ever complete and finished; the author are always two, one who writes the texts and another one--at least--who profits from its excess."
I bring up these things precisely because it's important to see that at least some of Borges's sources in The Book of Imaginary Beings are genuine. Interestingly, Borinsky lets us know that the Wang Thai Hai's ontological status is irrelevant. It's pretty much a straight translation, although the entry in the original does refer to the author, Ong-te-hoe, in the third person.
(a couple of translation notes: Giovanni uses "scarlet" for the eyes, but the Spanish uses "cornalinas," so I assume Hunley's is more strictly accurate; carnelian stones don't exactly look red to me; also, Giovanni's use of India ink isn't in the Spanish: "la tinta china")