If you write about malware, how many times a day do see something like this?

There’s absolutely no sound grammatical reason to consider the word “trojan”, in the context of malware, a proper noun.
This capitalization came from the original long-form, Trojan horse, which become shortened to “trojan”.
One would capitalize Trojan when using the compound “Trojan horse” in the context of malware, given that this is the use of the original proper noun in an analogous context.
If you agree or think otherwise, comment!

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