In Latin, bonus is an adjective, not a noun. The New College Latin-English dictionary does give noun definitions for bonus as 'good, profit, advantage' and as a plural (boni) 'goods, property'. (I suspect those entries are from metonymic usage from poetry, but as I am not 2500 years old, I can not say with certainty.) In any event, the Latin definitions do not match the English usage, and therefore the English usage is no longer Latin and so not subject to Latin declension. While language is defined by the people who use it, and the OP may wish to start a precedent, I will think of this and sniffle haughtily at him/her every time I see a Redcoat blow out a Praetorian.
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As for octopus, Webster gives the etymology as from Greek, oktopous, and if you check Plato or a Greek-English dictionary, you'll find octopous (οκτώπους
means "eight feet long, broad or high", and although octapous (οκτάπους
does mean "eight-footed", both are still adjectives and do not definitively denote the 8-footed octopus as an animal which is the usual English definition. However because some old fart, Linnaeus no doubt, decided to classify all the plants and animals with Latinized names, it is considered proper but not necessary to use the appropriate Latin pluralization rules when refering to them (e.g. octopi, cacti) especially when in a scientific context.