$HBO_2$ is metaboric acid (left pic.), boric is on the right pic.
$HPO_3$ is metaphosphoric acid (on the left pic),...
Actually, blame old nomenclature. Meta-acid has "one less $H_2O$ molecule" than ortho-acid. Thus, $HBO_2$ is metaboric acid, while $H_2BO_3$ is orthoboric (same about any other acid). But as you can see from pics, it's not really "water molecule" that differs meta-acid from ortho-acid. In case of metaboric acid, it's not even 1 H and 2 O's difference! So now ortho-acids are called simply acids (for example, boric), while meta-acids are still called meta-acids. Maybe because they are formed from ortho-acids. So, ortho-acids are considered "true acids", while meta-acids are, like, secondary. But I should mention that sometimes ortho-acids are called ortho-acids. I believe it depends on occurence of these substances in history of chemistry: the most "historically" important of two (or popular) has the right to skip prefix.
[remark] I know that metaphosphoric acid, as well as pic which I've given, is old fossil and now there is no such thing as "monophosphoric" acid (check for cyclo-triphosphoric acid, though). But the question was about this acid.
[edited]For borates, ortho-acid is almost always "boric acid" (rarely - "orthoboric acid"), and meta-acid is always "metaboric acid". So, (ortho)boric acid = H3BO3, metaboric acid = HBO2.
Phosphoric acid = H3PO4. It's very unlikely to meet name "orthophosphoric", because you have no metaphosphoric pair. Sometimes (in old textbooks/in poor-made school course of chemistry) you can hear "metaphosphoric acid", which is on the pic above, but the name and pic itself are false. HPO3 is now considered to be cyclo-triphosphoric acid, as I've said. As for other possible acids - some have "true name" for meta-acid, while ortho-acids are called ortho-acids, some are called ortho- and meta-, some are like phosphoric acids: there is no ortho- or meta-, only "true acid" and cyclo-acid. Specificity of nomenclature.
Googling for any good explanation now gave me this:
Good and detailed info about IUPAC nomenclature. Hope this helps.