In latin the word virus does not have an attested plural[1] but if we model it from the other neuter nouns of the second declension it would be "vira".

[1]https://en.wiktionary.org/wiki/virus#Declension_4

The plural of surplus is surpluses[2]. It would be *surplures/surplura in latin, so it is an English/French original. It doesn't make sense from an historical linguistic perspective to have a stem in dental "d" when in latin was in liquid "r": plus, pluris[3].

[2]https://en.wiktionary.org/wiki/surplus#Noun [3]https://en.wiktionary.org/wiki/plus#Declension

Cactus is a masculine name from the second declension: its latin plural is "cactī"[4]. Again, it would be unexplainable how that stem in dental would appear in a second declension name (stem in "o").

[4]https://en.wiktionary.org/wiki/cactus#Latin

Octopus is right, given that it's a third declension name with a dental stem.[5]

[5]https://en.wiktionary.org/wiki/octopus#Latin