Welcome @Greg_Deal !
Looking at the full passage, it just hit me that this fits the description of a crocodile. It is strong but also hides in the marsh. The only thing that seems out of place is that it eats grass but there are some carnivores that will eat grass to aid digestion. Grass can also just translate to herbage.
I found this article about some extinct crocodiles that were herbivores. You can view it in reader mode without subscription.
Plant-eating crocodiles thrived in dinosaur times.
Still, even modern crocs are not strict carnivores. Many species have been found to occasionally eat fruit, sometimes straight from the tree. And American alligators fed largely plant-based diets for a few months did not appear to suffer any negative health consequences. Clearly, crocodilians are more flexible than they are usually given credit for, and today’s crocs are far better adapted than the common misnomer “living fossil” might imply.