Sheep are able to escape from a farm because of ladders. SHEEP CAN'T CLIMB LADDERS! In the pictures, you can see the cells for each color of sheep. when the water is turned on, sheep are sometimes able to grab onto the ladder and get out. None escaped when there was not a ladder. There are 2 sheep per cell.
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Expanding on what Zenimia said, if a mob's pathfinder is trying to move towards a point above a ladder, or something is pushing it up a ladder (eg. water) then it will move up, but the pathfinder will not see the ladder as a more appropriate method than, say, walking against a wall. That means that it is incredibly rare that a mob will intentionally use a ladder, as it's not actually meant to happen.
Any mob can climb ladders, although their AI will not recognize it as a suitable method.