Unlike most reptiles, iguanas can be harness-trained, and a well-fitted harness is one of the more genuinely useful accessories for the species. It allows supervised outdoor time — and the natural sunlight that comes with it — without the risk of a fast, agile animal escaping. For an iguana that tolerates handling, a harness opens up enrichment options that a free-roaming animal simply cannot have safely.
Fit and design are everything. A harness should be sized for the animal and adjustable as it grows, secure enough that the iguana cannot wriggle free, but not so tight that it restricts breathing or movement. Designs made specifically for lizards account for body shape better than improvised alternatives, and the iguana should be acclimated to wearing it gradually rather than forced into it.
A harness is a supervision tool, not a substitute for a secure enclosure. Even leashed, an iguana outdoors needs constant attention, shade, and protection, because these animals are strong and quick and a leash can fail. Used sensibly, though, a harness is one of the few accessories that meaningfully expands an iguana's quality of life.