Insects may be smarter than you think
(15 minute TED talk on YouTube)Plants can perceive things
My personal belief is that any creature that (thoroughly) attempts to maintain itself and that has means to perceive things, i.e. any living being, can feel pain and possibly also fear. Of course the experience need not be the same for all species. I imagine that for a species with a slow internal communication system, like a plant, the experience is much less focused (diffuse if you want) and perhaps less intense as well. Still, it's there, because it has a function in the survival of the plant.
Now, a different question is whether other species have a conscious experience
of feeling pain. Currently it is so poorly understood where phenomenal consciousness comes from, that I think it is impossible to tell. However, it would seem very naieve to me to assume that only humans have phenomenal consciousness, or even that only "smart" mammals have it. A relatively straightforward reason why it may be advantageous to "dumber" species as well is, that it may help a creature to adjust its current behaviour to an experience when there is no direct (neural) shortcut between the percept and the behaviour.
question, is whether pain must be experienced consciously in order for suffering to be possible.