In any mode and any type of world, mobs of all sorts (mainly cows, sheep, pigs, chickens) can escape from fully fenced enclosures. This only seems to happen when the enclosure gets a certain amount of mobs inside, eg for an area of 26 by 14 (364 square) and the mobs get over a count of approx. 40 then they start to escape through the fences. Any questions just ask.