Passive Mobs such as Horses, Rabbits, any animal wont spawn at all after world border has expanded. I am on a vanilla smp server running the snapshots with a weekly expanding world border. New areas inside the border dont spawn in any animals, regardless of players or just standing around waiting for them.
MC-70330
Passive Mobs Don't Spawn After World Border Has Moved
Resolved
Incomplete
0
Can't confirm in 1.8.3. Are you still experiencing the issue? Be aware that passive mobs don't spawn naturally anymore, only when new chunks generate.