Beaks (owl, eagle, falcon)
Thorns (raspberry bush, rose bush, blackberries bush, brambles)
Venus Fly Trap
Aloe Vera Plant
Barbed wire fences (ranches, farms, prisons, mine fields, government areas)
Wrought iron fences
Spur, nub, quill, needled, bristle, thorny, horned, spike, briary, burry
Describing a shape is best done in as few words as possible. Think of the shape as a camera snap shot--you want to capture the gist of what you mean as soon as possible so you can get on with other related (and more important) detail, and the action happening in the scene
A weak example:
The mugger held his knife out, a long, curved blade with a spurred backbone, made for causing more damage coming out than it would going in. I handed my wallet over; the forty bucks I had in it wasn't worth losing my life.
What's wrong with this example?
The words chosen to describe the shape don't create a precise image.
A strong example:
The mugger held his knife out, moonlight glittering off a wickedly barbed edge. A huge blade like this had to be all show--it was a ridiculous choice for a back alley shakedown. Still, as he waved it back and forth, you couldn't help but imagine the kind of damage it might do. I handed my wallet over, the forty bucks I had in it not worth the risk of being wrong.
Why does this work?
This example puts more emphasis on how the shape causes a heightened emotion rather than over explain the look of it.