The Illustration is by the artist Russ Nicholson.
These two guard the gate to the Black Tower, AKA the Citadel of Chaos. The Ape-dog (ape's head, dog's body) does the talking, while the Dog-ape (dog's head, ape's body) provides backup with the aid of a mace (not shown in the image).
Both of these creatures are supposed to have been created by Balthus Dire's father, Craggen Dire. Neither of them have actual names, they are just referred to as Ape-Dog and Dog-Ape.
They were originally unique, although the AFF sourcebook Beyond the Pit by Andrew Wright opened this up again, pointing out that blogs (from Armies of Death) bear a strong resemblance to the Dog-Ape. Ironically blogs even possess the engorged belly and opposable toes of an ape, though they lack the body hair.
An Ape-Dog appears on an encunter table in the AFF book Creatures of Mishna by Benjamin Quinton-Bottley, although given that the Citadel of Chaos book doesn't have a set date, it's unclear whether this is a different Ape-Dog, or simply the same one seeking asylum or a holiday (I'd guess that it's up to the GM to decide).
As a final sidenote, notice that the door beyond the portcullis (look at those decorative swirls!) appears to be a singular door. Most gates like this are usually two that swing open but not this one, perhaps because it's already quite a small gate? Besides, the gamebook mentions that a side-door is opened for you anyway, so what the main door is used for is anyone's guess.
