Why is an octopus called an octopus? I get the “octo-” part, but why “-pus”, not “-pod”?
Nominative form of the (:atinized Greek) noun, borrowed. I’m not going to try to code the Greek alphabet, but it transliterates to oktopous. Latin borrowed it, changing the K to a C according to established Latin spelling usage and dropping the -o- in the ending. Quite a few borrowed Latin and Greek words are brought over in the nominative: rhinoceros and fax are two good examples.
It should also be noted that the correct Latin plural is octopodes, not octopi. The standard English plural is octopuses. However, so many people use the pseudo-Latin form octopi, under the impression that is the Latin plural, that it has even entered some dictionaries, despite having no real linguistic justification. In scientific usage, the group is frequently referred to as octopods.
It might also be mentioned that the correct Latin plural of rhinoceros is rhinocerotes, not rhinoceri. Better to use the standard English plural rhinoceroses.
See this masterful Staff Report by bibliophage for other example of English plurals of Latin words.
Isn’t “fax” just a contraction of “facsimile”? 
Facsimile = fac simile. Fac from facio(-ire), “to make”; the exact form is the (irregular) imperative singular, “[I command you to] make [something]!”, but is probably mainly used as a combining form. Simile meaning “same”.
I’m not sure how this relates to what was already written, but there it is.
My second (and erroneous) example of a Latin nominative borrowed into English was “fax”
It’d be cool if it were a direct Latin borrowing, though, because then we could use “faces” as the plural of “fax” (which, if it were a Latin word, would probably be declined “fax, facis”, in parallel to “pax, pacis”).
That’s how Caesar learned of the conquest of Gaul: fax Romanis.