Interesting. Maybe we've played it wrong that we've only used Pack Hunting with a Carnivore card (never without) assuming that a non-Carnivore doesn't hunt anything where body size is a determinant for possibility of the hunt.
You can play the trait on any species, but it provides essentially no benefit unless that species also has the Carnivore trait.
The reason I used the word "essentially" above is due to the one exception where it would provide *some* benefit. That is in the final scoring as spelled out on page 10 of the Climate rules and page 9 of the Evolution, 2nd Edition rules:
Each Trait Card on a surviving species is worth 1 point.