Certain types of animals and plants are called hardy. For example, we have native breeds of pony (horses) that live wild in the forests or on the moors. These ponies have to survive harsh conditions. They have to be hardy (tough/able to survive). Put a thoroughbred racehorse out on the moors and it would die! But a hardy pony will be able to find food and survive the cold and wet.

It's the same with plants. If you go to the garden centre to buy a plant for the garden it might be best to buy a hardy plant that will survive the winter frosts!

thank you guys who have been so helpful！！！ i really appreciate that. 

All my life I thought it was "hearty." MoiraWendy is correct. Her comment is your answer. "Hearty" is a word though that can have similar connotations. 

Hardy: Healthy, courageous, capable of surviving harsh conditions

Hearty: Exuberant, vigorous, substantial

You may mean "hearty animal", which is pronounced exactly like "hardy animal" but with a "t" instead of a "d". "Hearty animal" would mean "(strong, healthy, virile) animal."