The "Hokmuto" is the Winged Muto at the airport, it's called the Hokmuto because they raised it in Hokkaido underneath the nuclear facility that Cranston's character apparently worked at, in the script it was heavily altered, they exposed its egg to a ton of radiation to get it to hatch, and then fed it freaking plutonium.
I don't know if the characters in the script even call it that, the script just uses that name to differentiate between it and the other Muto, but I'll have to check what I saved from Snowdenzilla.