Maybe I'm biased because this is the exact setup I have for my Horses, but I'd put in a vote to keep your first idea for seahorses! They make such amazing pets, and a 37 column tank is perfect for 3 or 4 medium sized horses, or a pair of large ones.
You mentioned you were concerned because you would be going away for a couple weekends? Big deal! Put a timer on the light, throw a dozen live glass shrimp in the tank, and let them fend for themselves! Horses LOVE live feeder shrimp, and its amazing to watch them chow down on one. They really go to town hunting the shrimp down. Only thing is, see if you can find a LFS who keeps their feeder shrimp in salt water...acclimating fresh water feeders to salt water is a bit of a pain.
I'm just saying that horses are challenging, but less difficult to keep than some people think. PM me your email address and I'll send you some info if you want!