>>36047447ghost type refers to a pokemon having some degree of intangibility. koffing and weezing are solid (and not ghost *themed*), and as such are just poison types. gastly and haunter are gases (they're even classified as "the gas pokemon"), but are ghost-themed.
one can debate whether or not gastly and co. are more than just ghost-like animals but there is no debate to be had as far as whether or not they're poison types "on purpose".