Like the giant and mammoth fleas, here are two monsters that at first look silly, but are actually pretty great.
First of all, if you grew up with Super Friends like I did, then you have images of Aquaman riding an angry purple seahorse burned into your retinas…and that image is awesome. Thus, the giant seahorse. And hey, it can drag 8,000 pounds—perfect for the next time your PCs accidentally drop their favorite sacrificial altar overboard. (Look, it happens. I’m not saying how I know.)
As for the killer seahorse, I’ll just quote the Bestiary 4 directly:
Killer seahorses have been known to crush other creatures into a bloody pulp just before giving birth, ensuring a good meal for their newborns (called "fry"). A killer seahorse fry […] is an aggressive swarm feeder, like a piranha.
Um…yikes. Lesson for today: Never laugh at a monster because of its name alone, especially when that monster is a pregnant father.
A sea hag’s sisters taunt her for her love of giant seahorses. If both her sisters are alive she will fight with the rest of her coven. But if one of her sisters dies, she offers to help the adventurers under two conditions. First, they must leave her precious steeds alone. (Of course, the temperamental giant seahorses may attack the adventurers anyway.) Second, she also asks that they bring her both sets of her sisters’ eyes, though she will not say why…
The Trickster God is known by many names and shapes on land. He is the Raven, Br’er Coyote, the Laughing Monkey, and the Spider. But below the ocean, he wears other, even more unpredictable shapes: the color-changing, lock-picking Father Octopus, the tree-climbing Mangrove Crab, the smiling Gulper Eel who swallows prey larger than he is, and the Gravid Sea Horse. Undersea temples to the Trickster are rare—often the temples of other gods reconsecrated after they sank below the waves. A common feature of these temples is a trapped chamber that seals intruders inside with a killer sea horse and his ravenous fry.
Cultists of Dagon work to flood the canals of Calima. Adventurers will need to race from crisis to crisis to foil the saboteurs, which include half-fiend gillmen (see The Inner Sea World Guide), skum, mutilated minotaurs, and worse. In one vaulted passage, adventurers come across a servant of the demon lord of deformity feasting on one of their former adversaries. The bloated, pregnant monstrosity (actually a mutated killer sea horse) abandons its meal and extends its horrible tail to constrict them.
—Pathfinder Bestiary 4 234
Note that the seahorses are inexplicably not in the Bestiary 4 online Table of Contents but do appear if you input them in the search box.
Aquaman’s seahorse per The Big Bang Theory? Less awesome.