i was expecting someone to say that its because uranus controlls the elephants like posseidon controlls the sea kings, personally i really dont think thats the case, but ive seen people defend this idea before so im surprised noone has pointed it out yet.
i think that its because they arent really "animals", think about it, all the "normal" animals we saw so far can comunicate with chopper, but zunisha and sea kings cant, but what elese conects these tow semingly completly diferent kinds of creatures? they can comunicate telepathically, animals cant do that, they make sounds, sounds wich chopper can translate, perhaps the reason is that there is a skill wich is comunicating through telepathy, that some species as a whole have, but that only some humans, like luffy, momonosuke and roger have for some reason, this seems like the most reasonable explanation to me, zunisha is in a diferent cathegory of creature, the "telepathic" creatures, a cathegory wich fits the sea kings as well.