Imu does not use his full Zoan form; instead, he looks like a hybrid with characteristics of a Devil-Devil Fruit.
When Zoro and Gerd find Sommers and pick up his heart, Zoro wonders if Sommers would die if he destroyed it, whereupon Sommers becomes very scared and shocked.
When Imu lands on Elbaf, he spits blood and falls to his knees. He then transforms, and we finally see his design.