The correct answer is - transitional form.
The transitional form in the animals is the form they have while they undergo changes, usually more extreme while adapting to live in total different environment. The Tiktaalik for example, is a species that looks like a mixture of fish and amphibian. The reason for that is that this creature was evolving in order to be able to change the water environment for the terrestrial environment. These two environments are totally different and they require different shape, behavior, organs, etc. Since this animal was in a phase where it was able to live in the marine environment, but not yet totally prepared for the terrestrial life, thus moving using both environments, it had both adaptations, the original ones, and the new ones for the new environment.