Shadow ogres are not randomly generated; they appear only in Grund's Stronghold or possibly via polymorph. This may be due to an oversight, as they are not actually marked as not being generated, but lack a frequency altogether. Normally, monsters that lack a frequency but do not have the "not randomly generated" flag can still appear in aligned branches of the dungeon; however, shadow ogres are also marked as appearing only in Gehennom, which is unaligned, so they will not appear there either.
Shadow ogres can be fairly dangerous: they are faster than an unhasted player, and hit fairly hard. If you lack magic cancellation, their life-draining touch can be devastating, although less so than those of a star vampire. They lack any resistances, so attack wands are a good option if you feel threatened.