I'm still not sure how tf to parse the second sentence. Reading it the same way as the 1st sentence is intended to be read, I get:
"Rodents, including the capybara, the world's largest, sometimes exceeding 100 pounds are numerous."
At first I read it as capybaras are the world's largest rodents, and ones exceeding 100 pounds are numerous in this area. Assuming that's the correct reading of it, I don't know that there's a better way to phrase that in the space limitations of the sign.
The more I look at it though, I think it's saying rodents are numerous, including the capybara (the world's largest rodent), which sometimes can exceed 100 pounds.
Either way, I hate this sign.