I am not an expert on the subject but I have dealt with EtherCAT for industrial controllers and have vague plans to one day using it with a Raspberry Pi. As far as I am aware, EtherCAT masters use standard Ethernet hardware.
That said, the main benefit of EtherCAT is it's speed for realtime systems. Safety, motion control, etc. As Jamesh mentioned, I don't think Raspberry Pi can take advantage of that by itself and you would need extra hardware.
I have not looked over the whole site , but there is some interesting stuff over here:
http://www.simplerobot.net/2018/06/rpi- ... picat.html
I believe they are only using extra hardware for the realtime portion of EtherCAT.
If you want to make a non-realtime master then look at SOEM (Simple Open EtherCAT Master):
If you want to make a slave you will definitely need specialized hardware.
Again, not an expert. Maybe there is some helpful information here.