Werewolves are common in fantasy or horror stories I'm familiar with, so are werecats and werebears.
The shamanistic belief that humans can become animals and change back to human, technically called "lycanthropy" (but if you know Greek roots you'll see the word "wolf" in there) has been applied to various animals in various cultures throughout the world in ancient beliefs but also in stories of more modern times. Almost always the "were" creatures are large land mammals (though island cultures do include some sea creatures).
So in Africa, where I happen to be at the moment, there exist beliefs in what we could call "were-" jackals, giraffes, lions, hippos, and elephants in traditional animistic religions. Now, there probably exists a story about a "werelephant" somewhere, but I've never heard of it. It certainly isn't common, so since this blog is always looking for new or uncommon story ideas, there might be something worth mining right there--African "were"beasts within the setting of a new tale.
But let's take this further--how about domestic animals? Were-horses, cows, dogs, hogs, or donkeys, or even were-chickens or turkeys could make for an unusual story twist, right?
Let's go further still: How about a story about someone who turns into a crab at the first high tide after the full moon? And doesn't turn back until the first low tide after the new moon? Or something similiar? Stories could be crafted about werecrawfish, weredolphins or whales, werebats or birds, or even wereflies or werelice. Why restrict this to the big animals? We're talking speculative fiction anyway, so any creature should be fair game.
You could even go as far as were-trees or were-grass. But I think it would be challenging to write an interesting tale about someone who turns into a tree from time to time...unless she could still think and move, even though a tree...