What are some highly intelligent mammal species that are true herbivores?

The smartest mammals I can think of aside from humans are mostly omnivores (dogs, pigs, chimpanzees) with some carnivores (dolphins, cats) and the only herbivore I can think of that's considered intelligent is the elephant. Are they smarter than say cows or rhinos or kangaroos?

Answer

Haven't anybody mentioned Orangs so far?. They are mainly frugivore. 

We do it better in recognizing intelligence in species that remind our own behaviour in some way. I would add that lay people tend to overrate carnivorous animals, due to simplistic interpretation of them as "winners", and that behaviour researches tend to make experimets baised towards social animals.
Thus, hervibores, solitary animalsand those that mix these two characteristiques are perceived as less smart.
There is this funny video that somebody shared with me of a cow opening the mechanisms that trap them when eaten in line feeders in milk farms (and excuse me, I don't know the exact translation of these terms, but if you see the video you'll understand what I say and have a fun time, especially at the end of the pic. Notice that the cow know how to open the thing independently wether they trap her or a fellow, using different techniques). 

Cows and goats are true hervibores, ruminants, that's why I selected these species. Somebody could argue that they ingest a lot of insects with their grass...but, let's be serious, those animals evoved their stomachs to digest plants efficently.