Water Breathing. The seahorse can breathe only underwater.
Ram. Melee Attack Roll: +4, reach 5 ft. Hit:9 (2d6 + 2) Bludgeoning damage, or 11 (2d8 + 2) Bludgeoning damage if the seahorse moved 20+ feet straight toward the target immediately before the hit.
Bubble Dash. While underwater, the seahorse moves up to half its Swim Speed without provoking Opportunity Attacks.
Use these stat blocks to represent the creatures they're named for or other similar creatures. For example, the Panther stat block can also represent a mountain lion, while the Giant Goat stat block might represent a buffalo. Any of these stat blocks might also serve as fantastical animals with distinctive names and cosmetic details unique to your D&D adventures.