Here are some possibilities depending on what you're thinking:
* The next letter in the alphabet: T
* The next animal in a marine food chain: Depending on the specific seahorse, it could be a predator like a larger fish or a sea turtle, or it could be something that eats the seahorse's food, like a smaller fish.
* The next thing you'd see on a dive: It could be anything! It depends on where you're diving and what kind of environment you're in.
Let me know if you had a specific idea in mind, and I can help you figure out what comes after a seahorse in that context!