You are wasting time computing the actual distance. Instead of getDist(), create a getDistSquared() function, and compare against the square of the vision distance.
creatureViewDistSquared = creatureViewDist * creatureViewDist
...
if getDistSquared(creature.pos, otherC.pos) < creatureViewDistSquared:
...
But first, do the partitioning as suggested by maxb.
If you maintain a heading vector for each creature (dx,dy), you could “dot” that with a displacement vector to the observed creature. If the dot product is negative, the observed creature is more than 90° away from the heading/viewing direction. This will, on average, remove more than half of the creatures.
If the heading vector is normalized, the dot product would also need to be less than or equal to the viewing distance.
This will give you a fast filter, using only two multiples, two subtractions, and an addition:
dot_product = c.nx * (otherC.x - c.x) + c.ny * (otherC.y - c.y)
if dot_product >= 0 and dot_product <= creatureViewDistance:
....
It occurs to me, you should also do the trivial reject first.
dx = otherC.x - c.x
dy = otherC.y - c.y
if abs(dx) > creatureViewDistance or abs(dy) > creatureViewDistance:
continue
if dx*dx + dy*dy > creatureViewDistSquared:
continue
dot_product = c.nx * dx + c.ny * dy
if dot_product >= 0 and dot_product <= creatureViewDistance:
....
At this point, you can do the angle calculation, to ensure the creature is in the 60° limit