This is a matter of style, so you should follow the recommendation of your style manual, either the one you've adopted or the one thrust upon you. I use The Chicago Manual of Style, which mandates that asides, nonessential interruptions, be set off with commas. This is also a kindness to your readers, who won't be tempted to look for a parse like "rhinoceroses for sale."
While you're being kind, you might as well protect your readers from expecting a list of three things -- (1) vegetarian animals, (2) giraffes, and (3) rhinoceroses.
Also note that "vegetarians" is distracting here because the word almost always refers to people who have made a dietary choice. Animals that don't eat meat are called herbivores.
Putting all these kindnesses together results in
Other herbivores -- giraffes and rhinoceroses, for example -- help as