This is a really good question Paige! Like you said, sometimes flares have a loop-like structure and sometimes they have a arch-like structure with a cusp at the top. Flares are a result of an interaction between magnetic field lines called "reconnection" where two field lines sort of join together. This joining releases a great amount of energy which can be seen tracing out the newly formed magnetic field lines. So, the shape of the flare depends on the shape of the newly "reconnected" field lines and also at what temperature you are observing the flare. Also, sometimes people get flares confused with prominences, which are structures of cool material that also show a variety of shapes including loops and arches.

Hope this helps!

Holly