I love classifying activities! They generate great discussion and real interest amongst the children which invariably leads to new and authentic work.
Classifying animals into herbivore, carnivore and omnivore can generate some excellent discussion about animal features and adaptations and can lead to discussions about the food chain.
There are a multitude of fantastic materials out there, free and paid. For many of my zoology classifying activities I have the collection of animals of the continents cards from Imagine Our Life. I like that they are designed to go with Safari Toobs, which you may wish to incorporate for younger students if you own any of the sets of Toobs. I also like their size for this work as classifying work can take up a lot of space and some of my other sets of images for zoology would take up the whole floor! By having a the complete set of these cards it means I am able to mix and match as required and have an interesting array of animal cards on hand.
When we did this activity recently I used the animal cards for Africa, Asia and Australia. Depending on your students you could add more or less cards, or have all sets on the shelf as an extension.
I made heading labels with the words "herbivore", "carnivore" and "omnivore" with the definitions on the reverse side. You can find my file HERE.
We had some great discussion about which animals belong to which group - some insisting on doing some research to end the debate over a couple of the animals.
We placed the animals that the children were sure of under each respective heading and put the ones that they could not agree on along the bottom until the children did some further research.
Afterwards the children wrote the definitions in their zoology books along with some examples of each.