Since it's impossible to make Hippocampi stay wander or follow, they spawn next to the wayward lanterns. It's prolly the same with deathworms. I suggest adding them to the pool of mobs that can be ordered to stay wander or follow with Domestication Innovation installed. (I'd maybe add horses to the pool while we r at it)

(Just look at this cute little boi on unforgivably dry land, away form sea)

I second this, especially with how prone Hippocampi are to swimming off the moment you leave them unattended