Steinback is correct that the only officially WR Pentax lenses ar the DA* and WR lenses. For example, there are at least 3 versions of the 18-55mm kit lens: DA 18-55mm, DA 18-55mm L and 18-55mm WR. The latter was released about the same time as the K-7 (I believe) and is weather resistant.
However, some other Pentax and 3rd party lenses (eg metallic primes) are very sturdy and would be very close to WR. See the story and photos of
https://www.pentaxforums.com/forums/pentax-dslr-discussion/93367-k-7-steve-kr...gua-6962m.html (He took the DA18-250mm to some very rough place.). My Voigtlander 58mm is built like a tank and I take it often outdoor to rough places (rain, mud, storm) and have had no problem.
Let me add for completeness that WR does not mean waterproof. WR is more like some form of insurance against accidents and incidents. Most Pentaxians would regularly clean/wipe carefully the camera and lens if it becomes wet or dusty, even WR camera and lenses.
Hope that the comment will help...
PS: the Nikon D90 is not rated WR. I looked very carefully at the seals, doors, connectors of the body when I was considering it... The D90 is definitely not WR ,and would not take a D90 to places where I take my K-7.