When any mob is killed and they enter their death animation, they cease everything they do. This doesn't apply to horse-like mobs for some reason as they will still turn and look around during this animation and also will face the player if nearby.
To replicate this, use one of the summon commands below and watch as the horse will turn and face you or just look around as if it were still alive.
/summon horse ~ ~ ~ {Health:0f,DeathTime:-200s}
/summon skeleton_horse ~ ~ ~ {Health:0f,DeathTime:-200s}
/summon zombie_horse ~ ~ ~ {Health:0f,DeathTime:-200s}
/summon donkey ~ ~ ~ {Health:0f,DeathTime:-200s}
/summon mule ~ ~ ~ {Health:0f,DeathTime:-200s}
/summon camel ~ ~ ~ {Health:0f,DeathTime:-200s}
You could compare this behavior with another mob like a pig in this following command.
/summon pig ~ ~ ~ {Health:0f,DeathTime:-200s}
You would see that the pig doesn't move around like the horse mobs and acts just like any other mob does when it is in its death animation.
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This isn't a duplicate of MC-19757 as the horse mobs will move and look around on their own during the death animation because I believe they don't have any code for them to stop this behavior as if they were still alive compared to MC-19757 as to where a mob that is turning before it dies continues to turn to that position until they are there and won't turn anymore compared to the horse which has a mind of its own.
Not too sure, but I believe this is a duplicate of MC-19757.