The Traveller RPG system has a major herbivorous race, the K’kree.
Asimov’s short story “Hostess” features vegetarian civilizations prominently. In the story there are four or five other intelligent races in the galaxy, all of whom evolved from grazing herbivores. Mankind is considered to be a very strange anomaly for being omnivorous, as well as for several other reasons (dying so early–most other races grow at an infinitesimal rate their whole lives, and live several centuries at least; sleeping/dreaming; and some others I won’t give away).
Here’s an idea: Most of our primate ancestors were omnivores. But genus Homo didn’t begin to develop the level of intelligence that distinguishes us from chimpanzees until our ancestors developed full bipedal gait, thus freeing our hands to be used as manipulative organs. (http://en.wikipedia.org/wiki/Human_evolution#The_Homo_genus) And that stimulated the development of the intelligence necessary to make full use of them. Language – full symbol manipulation ability – evolved still later; apparently it would have been impossible without the neurological groundwork being laid at the earlier stage. And bipedal gait was prerequisite for all of this, because our ancestors – like all land-dwelling vertebrates – had no more than four limbs, and most of them used all four for walking or brachiation.
Suppose there were a planet were something similar to land-dwelling Earth vertebrates – all of them, from salamanders on up – had six limbs? And yo only need two to walk on, strictly speaking. Most vertebrates on this world would be like centaurs – with two limbs freed for manipulation, without the highly unlikely and rare combination of adaptations that makes bipedal locomotion possible. On such a world, intelligence might have a chance to evolve in any ecological niche – herbivore, carnivore or omnivore – where finely-tuned hands might offer any survival advantage. It might be a world with numerous sentient or near-sentient species.
Sorry, I mean, “And you only need four to walk on, strictly speaking.”
Parrots run the gamut from largely vegetarian to true omnivores. I don’t think that any are purley vegetarians. Most will eat insescts when available while the larger cockatoos aggressively seek out insect food and reportedly eat nestling birds and nest young of rodents as well. Keas are by far the most omnivorous and will scavange carrion and habitually rob the nests of other birds for eggs or young.
Hmmm. Interesting.
If anyone is interested, I’d like to expand the conversation.
In herbivore civilizations, what would be done to the predators? Would the herbivores drive them to extinction? Either purposefully or unintentionally?
This might be a whole other thread, but what do you speculate are the necessary attributes for a species to achieve space flight? Intellligence, ability to manipulate tools, social structure instead of solitary…
They’d probably be marginalized, as we have driven to restriction (and often near extinction) most predators that could threaten a human. However, it should be noted that no known predators, existant or extinct, are thought to have predated primarily on h. sapiens, and most predators will actively avoid people if other options are available.
This is assuming a race with human-like prerogatives, though. Maybe an intelligent race would be structured like social insects, with only the most intelligent in charge and warriors and drones sacrificing themselves to protect the alphas. Or maybe they’d maintain a balance with predators as an alternative to interspecies competition. It’s tempting to apply human lessons to a hypothetical alien species, when there are really many unforseen options.
A few random thoughts:
[ul]
[li]Vision (of some wavelength), as other senses are limited in distance of perception.[/li][li]Fire-based technology and terrestrial habitation. Octopuses may be very smart but they aren’t going build permenant structures or bang rocks together to make sparks.[/li][li]A conceptual curiousity (imagination) of the natural world and ability to form large social, bureaucratic units that are capable of appealing to individuals to work for a greater, long-term goal. [/li][li]It should go without saying, an analytical intelligence that allows for the creation of systems of mathematics, engineering, and general integration of knowledge of diverse physical sciences into a cohesive formulation of cross-discipinary theory. Liberal arts majors aren’t going to get us to the moon, even if they write beautify poetry about it.[/ul][/li]
Stranger
Or perhaps the predators woulde be objects of awe & terror, carefully preserved by a cynical priesthood, & worshiped as gods. 
Another take on herbivorous sentients in SF:
In James Hogan’s Giants trilogy, the dominant species–in fact, all land-dwelling species–on one planet was strictly herbivorous. Early in their evolution, a certain species of fish developed a modified circulatory system that made them poisonous to predators, among other advantages. The biodiversity was already quite low on the continental shelves (by our standards), and the species quickly overwhelmed all competition, then speciated to fill various niches, conserving the successful trait. From there, they spread onto land. Non-poisonous species (and the predators that fed on them) survived only in the ocean depths.
The poison was also the impetus for the development of intelligence on the planet. The animals were poisonous to themselves as well; trauma could cause toxins to leak over to the primary circulatory system. The ability to avoid injury was a key evolutionary advantage. The various strategies for this weren’t detailed, but one approach was obviously the development of sufficient intelligence and social structures to foresee and avoid accidents (although the intelligent species also had vestigial armor plates). The resulting sapients had a strong tendency to cautious, methodical thinking mixed with curiousity (associated with finding and remembering possible hazards).
As for their views on predators, well, their only knowledge of predation before developing spaceflight was of the predators of the deep ocean. Insofar as they had a concept analogous to Hell, that was it–a cold, dark place where monstrous creatures ate each other. Predation was regarded as an evolutionary dead-end, with predators condemned to stagnate in the most miserable place in the world.
In one of Jack Vance’s books (I think it’s The Book of Dreams in the Demon Prince series) he writes of a group of dedicated vegetarians who colonize a far planet in order to establish the perfect society.
When other humans arrive centuries later they find the vegetarians grazing in dumb herds. They can, with proper training, be domesticated as servants, and the females, if distracted with some vegetation to gnaw on and approached with care from behind, will submit to sexual congress.