Alternate Adult Forms:
I'm being open to your original goal, which is to explain a giant caterpillar as a developmental step in a species. So rather that an in-between step, why not simply an alternate end? I think this is close to what you were seeking in the original question, but also meets this question's needs.
Like the neotene Tiger salamander, which can have an aquatic axolotl-like adult version if terrestrial conditions are poor, Your chasers are not an intermediate step of development, but an alternate end-point taking advantage of differing environmental conditions to survive and exploit differing conditions.
I am not entirely sure what the ideal goal end-adult is for your species - a giant terrestrial non-flying butterfly, or an actual giant butterfly. But for our purposes, it doesn't matter, as long as the chaser and traditional adult have different food sources and environmental niches. If, for example, a larval caterpillar is more omnivorous, consuming all food in the environment, while the butterfly form is dependent on following herd animals seasonally, failure of the herds to arrive in time could cause the larval form to become the chaser form; fast, able to prey on different prey or supplement with plants, but perhaps unable to undergo the long migrations normal for the butterfly form.
Or perhaps your butterfly form is dependent on the emergence of giant flowers. Only you know your ecosystem. But the chaser is simply a response to a variation in conditions that lets an otherwise niche-limited species survive change and adversity.