When people saw this monster in the 3e Fiend Folio, many of them probably thought "what a crappy monster" and they are right. This creature was something interesting that got turned into a shark that walks somehow without legs. So here is some history on the terlen. Hopefully this will inspire your own creations as well as making the monster more useful.

It was based on two different Gamma World monsters. The fleshin was a large, carnivorous fish that could fly. Not glide, but, at least in 1e, could fly for hours at a time. It could shapeshange into a lizardman like creature and its spines were horribly toxic. The name came from another fish, the terl (in later editions, the terleen), a feathered baracuda that flies via telekinesis. The feathers provide immunity to lasers and heat. It roosts in trees but nests in water (its larvae have working gills). It also has three attacks- its bite, a sonic blast and cyrokinesis (so mildly mutated for early GW).

Most of that went away when one author came up with the terlen. It was found in the 2nd Monstrous Compendium for Planescape and this is where is became much more interesting, at least to me. It is a fish that was native to a material plane and transfered to one of the central lower planes (Carceri, Gehenna or the Grey Waste). It was a shark or skate that adapted, or was magically altered, and thrived so well that it spread to the other two planes. It was a fish that could survive out of water, but it didn't walk. Its movement rate was 3 (so 1/4th that of an unencumbered human) and it could fly but not terribly fast (15). In fact, looking at its stats, the enlarged fins actually make it a slower swimmer than its ancestors (15). Why would it need to flop on land and fly? Look at its habitat- water ways filled with fiendish monsters. It adapted to predation by fleeing on land and in the air. It also gained the ability to hunt on land and in the air. Small, slow animals that come to drink or live near the water are easy prey. Flying allows them to see smaller aquatic critters easily from the air as well as an ability to eat slower fliers. Their home is slowly affecting their behavior [their alignment is Neutral (evil)] but hasn't infused them so much that they have become fiendish, yet.

In the FF, all the description on what the terlen is and how it came about was deleted or badly trunicated. Its movement stats were made identical (60' for all 3), which is nonsense. And the art was silly. The art in the MC wasn't great but at least it gave the impression of a flying fish that could move on land poorly (it is almost eel-like). The FF winged shark doesn't in the least. How does it move faster than a human without legs? It can be used, but without its history, the terlen becomes something less than it should be. An encounter (or several) that forces the DM to come up with a reason for their existence, odd appearance and strange movement.

The terlen, leomarch and quill; all from the PS MC II; are all prime material animals that have adapted to living on other planes, acting as predators and/or prey. In of themselves, they may not be that interesting. But they are the kernal of an idea that has so much potential. When you use other planes, consider those animals and plants that may have gotten there through Gate spells and how they adapted to local conditions. Or not, as the case may be. There is no reason that rats must be able to thrive in the gears of Mechanus. They may simply be there for a single or few generations and then die out. The description for the leomarch has an inkling of my Biology for Fantasy Settings article in d Infinity 4 (adapting creatures to magical laws and ecologies).

Too bad the Planescape authors didn't take it to heart.