However, the next most popular interpretation is that an insect must have wings as well as six legs. This definition of insects would make Giratina an insect.
Although many animals generally regarded as insects, such as mites, ants, and silverfish, don't have wings at all. A more accurate definition might be that an insect must be a six-legged arthropod, being defined as an animal with a hard exoskeleton and a segmented body. Given that it has skin literally made of rock, and at least two defined segments that we can see (head and body), this would definitely make Graveler an insect.
You might argue that none of these can possibly be insects because none of them are Bug-types, and while this is true, many Pokemon that are based on real-world insects are not Bug-types either. Vibrava, for example, is very clearly based on an antlion or a dragonfly, yet no part of its evolutionary line has the bug type (despite being part of the bug egg group).
So where is the line drawn? Is Vibrava an insect? Is Graveler an insect? Are Giratina or Machamp insects? Is the very word "insect" merely a social construct that can never be truly defined?