Goblins don't look in 8 directions. The game simply updates to picture depending on which way it last moved.
It's generally a good idea to think of how easy it would be to add to DROD. I agree it would be a cool monster, but it may be too difficult to add.
The problem is monsters are killed (except for snakes) if they are in the same square as Beethro's sword. I don't know how the code is set up for this (I hope it's a routine owned by the monster, but probably not) but it's probably a trivial thing to check if it's facing a particular way. The problem comes when we ask if Beethro is directly behind it - you'll notice that roaches won't move orthoganally (not diagonally) if they aren't in the same row or column as Beethro. They know where Beethro is, but not which way he's facing.
An alternative: The Behemoth will go for the nearest creature (it won't attack roach queens or snakes) and kill it, irrespective of whether it's Beethro or a monster. It will change its mind if a creature is nearer to it than its original target and go for it instead. If given a choice, it will go for the same kind of monster it ate last time, in the direction with the most creatures.
It stays in place on the turn it decides which monster to eat, or when it changes its mind.
Behemoths cannot eat other Behemoths, but they can try.
It cannot be killed when its target is Beethro. Therefore, to kill it, Beethro must approach it when it is one or two squares away from a monster it is intending to eat.
This is fairly similar to what you had in mind, but it's easier to implement (I hope). What does everyone else think? I'd prefer the first one, simply because we don't have enough monsters that make use of directions.
____________________________
What do you call an elephant at the North Pole?
Click here to view the secret text
×Lost.
/dy