According to the 3.5 monster manual –
Ettins like to establish their lairs in remote, rocky areas. They dwell in dark, underground caves that stink of decaying food and offal. They tolerate other creatures, such as orcs, if these can be useful in some way.
Otyughs are grotesque subterranean monsters that lurk within heaps of refuse. Although primarily scavengers that can eat almost any kind of refuse, they never object to a meal of fresh meat when the opportunity presents itself.
An otyugh spend most of its time within its lair, which it keeps filled with carrion, offal, and all manner of similar trash. An otyugh usually covers itself with this vile stuff, leaving only its sensory stalk exposed (the stack also carries the otyugh’s olfactory organ) and lies there for hours, shoveling food into its mouth.
Intelligent subterranean beings sometimes coexist with otyughs, which they regard as convenient garbage disposals: They dump their refuse in the lair of the otyugh, which generally refrains from attacking them.
Based on the rules as written would it be far fetched to think that these two creatures would live around each other?
I was specifically thinking of using the otyugh as a kind of guard-dog for the ettin.