They spawn without a raider and don't burn or despawn, despite changing the config. Also spawn rate is definitely too high.

Tested on my end and it works fine. Zombie horses do burn in daylight and spawn rate is not controlled by my mod. My mod only converts already spawned mobs into "rough" mobs.

Control hostile horse burn during daylight in config:

    # Set this to false to prevent undead horses from burning in sunlight (as long as they have no rider) [default: true]
    B:hostileHorseBurn=true

Control zombie horse spawns here. Set to 0 to disable. Otherwise, the higher the value the less frequent they spawn.

    # Chance (1 in X) that a zombie spawns riding a zombie horse
    # Set to 0 to disable this feature [range: 0 ~ 32767, default: 10]
    I:zombieHorseChance=10