From the Super Mario Wiki
Tadpoles are jumping enemies from Wario Land 3. Despite resembling tadpoles, they are described as "fish". They attack similarly to Tobī from Wario Land II, except that they are visible in the water, or sometimes lava, before they are ready to leap. They also have the added effect of causing Wario to roll, instead of simply bouncing back, and can only stop once he hits a wall. This can cause a large amount of walking to need to be redone. They are also larger than Tobī. They first appear in The Big Bridge.