"Ixthus" or "ichthus" is some kind of secret acronym for Jesus Christ. It is spelled with the Greek letters iota (i), chi (x or ch), theta (th), upsilon (u) and sigma (s), which forms an acronym for the Greek title "Iesous Christos, Theou Uion, Soter" which translates to "Jesus Christ, God's Son, Savior"
The word "Ichthus" is a Greek word for fish, and that is how fish came to symbolize Jesus Christ.
"Ixtus" appears to be a misspelling or a typo. It does not correspond to any known person, entity, or term. Can you provide more context or clarify your question?