Minecraft wiki entry for Development_versions, associated text: Mobs now pickup items and equip armor. (MCPE-14211)
Tamed horses can no longer be tempted by food that cannot be used to breed them.
Some mobs had a large delay for finding their nearest attackable target (Wolves attacking sheep).
Squid now spawn more frequently. Ocean cap is now 4 and river cap is now 2. (MCPE-39758)
Iron golems now ensure they can reach a target before choosing to attack it. (MCPE-42402)
Jumping animation is no longer broken for horses.
Polar Bear no longer chases player on its two back legs.
Sheep no longer eat grass through partial blocks.
Fixed issue where Enderman would not attack the player when looked at.
Mobs no longer start shaking when entering minecarts.
Fish no longer spawn in flowing water.
Despawn rules have been changed to closely match Java despawn rules (MCPE-21856)
This change allows mobs to more consistently spawn around the player and improve the frequency of mobs spawning underground while the player is exploring caves, as well as trying to farm areas that spawn specific mobs (eg. wither skeletons, in nether fortresses).
Any mobs that have been set to persist (tempted, name-tagged, tamed, bred, etc) never naturally despawn.
Almost all naturally spawning mobs automatically despawn when they are 54 blocks or further away from the nearest players.
Fish automatically despawn at a max range of 40 blocks or further.
If mobs are between 32 and 54 blocks from the nearest player, they must not take damage for 30 seconds as well as successfully roll a 1 in 800 chance to despawn.