Foxes don't stop moving outside of a 40 block radius around the player, like other mobs do.
I don't know if that's a bug, but it seems like a memory drain to me

To recreate, create a flat desert world and wait a minute.
Then spawn in som foxes to kill the bunnies.
After they killed them all (very entertaining) they will wander about, until they wander out of renderdistance.