Step 1: Understand the clue.
The question names a larva, Ammocoete, and asks which adult animal it grows into. So we need to recall which animal has this larval stage.
Step 2: What Ammocoete is.
Ammocoete is the small, blind, filter-feeding larva that lives buried in stream beds. It is a classic example studied because it shows primitive chordate features.
Step 3: The adult it becomes.
The Ammocoete grows into the lamprey, whose scientific name is Petromyzon. So Petromyzon is the right answer.
Step 4: Why the others are wrong.
Myxine is the hagfish and does not have an Ammocoete larva. Scoliodon is a dogfish shark and Hippocampus is the sea horse, both true fishes with their own development, not lampreys.
Step 5: Conclusion.
Therefore the correct answer is "Petromyzon". \[ \boxed{\text{Petromyzon}} \]