I don't like the idea of crabs - how would they be used?
However, flies I like. They'd move either on the diagonal or the orthogonal (like a snake), and it would switch every turn. So if a fly was two squares away from Beethro, it'd wouldn't be able to kill him in two turns like a roach. If you introduce roaches and flies together, you have problems as the roaches would try and queue-jump.
This of course raises the problem of which way do flies move if they can't move directly towards Beethro that turn? My suggestion is that counts for north, south, east and west are taken, and each square is valued (similarly to the code already in placed for goblin movement), but the criteria is the total amount of moves minus the relevant sum of movement counters. So if the fly was evaluating moving northwest, say, it'd add up the amount of north moves and the amount of west moves, and subtract that from the total. This would be added (maybe 5 times) to how close it is to Beethro, like a roach. Maybe it has a tendency to flee from the sword, so if it's a dead heat, it'll run away.
The tendency, for those who feel woozy from maths, is that the fly would move towards Beethro, moving so that its movement averages out in the end. Possibly if Beethro's on the other side of a room, the fly will start to get bored and wander off using these maths, but then that's another challenge.
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