Yes it is reasonable to apply 'rare' to one of a kind. For two reasons.
First is that it is reasonable to consider that "unique" is just an extreme form of 'rare'. But being an extreme form of rare does not stop it being rare. If there were only three of something it would be rare, or if there were two - therefore it does not make sense to everybody that ...
TLDR: the distinction actually exists in modern-day French, and this may be the source of the restriction of venom and venomous to injected toxic substances.
Walker's pronouncing dictionary from 1828 has the definitions
Poison: that which destroys or injures life; venom.
So it appears that, in the common language at least, there was no ...