Dogbane is a plant that so strongly resembles milkweed that many people, including those experienced in the field, can be fooled it it. Monarch caterpillars will not eat it.
Dogbane is related to milkweed and the Milkweed Tussock Moth (actually a tiger moth but more often called tussock moth) will eat it.
Dogbane also has white sap that can harm eyes, making it even more difficult to tell them apart. Click on this sentence to learn how to tell them apart.