Mammals and many other groups of animals usually transport oxygen using haemoglobin and other complex proteins, the core of which is based on an iron coordinated to a porphyrin. There are plenty of subtle differences in the protein structures but are there any notable alternatives to that central iron-porphyrin structure?
Obviously there are other chemicals that can do a similar job. Polyfluorinated hydrocarbons were once considered as a viable emergency alternative to donor-blood as they are very good at transporting oxygen. But those are not something that living creatures are likely to develop.
So the question is are there either possible or known alternative chemistries for transporting oxygen in living creatures?