We don't have synonyms for octapus yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are octopus and octupus.
(n) :
Misspelling of octopus. [(countable, loosely) Any of several marine molluscs of the order Octopoda, having no internal or external protective shell or bone (unlike the nautilus, squid and cuttlefish) and eight arms each covered with suckers.]
(n) :
Misspelling of octopus. [(countable, loosely) Any of several marine molluscs of the order Octopoda, having no internal or external protective shell or bone (unlike the nautilus, squid and cuttlefish) and eight arms each covered with suckers.]