These tags are also meant to describe "unique" environments, and not overly-generalized environments like plains, grasslands, and coastal areas.
- aerial - Represents creatures which you would (or could) normally find high in the sky.
- aquatic - Represents creatures that would be encountered on or below the surface of water.
- arctic - Represents creatures that would normally be encountered in snow cover, icy, polar, or arctic areas.
- desert - Represents creatures that would normally be encountered in deserts and other barren landscapes where plant life is scarce or non-existent.
- forest - Represents creatures that you would normally find in the wilderness, and includes all types of woodlands and forests.
- hill - This is a contentious tag for me, as I believe most "hill" monsters should instead be given the "mountain" tag. However there are a few creatures that based on their game description are only found in hills, so for those few creatures, please apply the "hill" tag.
- jungle - While this tag could have been included within the forests category, I think the hot humid conditions of a jungle, and the uniqueness of the creatures that dwell there, certainly justifies this separate tag.
- mountain - Represents creatures that you would normally find in the higher altitudes of hills and mountains.
- subterranean - Represents creatures that would be encountered below ground and/or dwell in caves and caverns. D&D's "Underdark" realms, and Pathfinder's "Darklands" would be included as areas described by the "subterranean" tag.
- swamp - Represents creatures that would be found in any sort of wetland area which would include swamps, marshes, bogs, and fens.
- volcanic - Represents creatures that would be found living within volcanos and includes all lava and magma based creatures.
While I don't think there is a need to identify specific planes of origin, I believe there is merit in denoting that a creature is not from the material plane, and for iconic good and evil creatures, to denote if they are from the upper or lower planes.
- extraplanar - Represents creatures that originate from a plane of existence other than the Material Plane
- lower planes - Represents creatures that originate from planes that are associated with evil-natured denizens, such as demons, devils, and yugoloths.
- upper planes - Represents creatures that originate from planes that are associated with good-natured denizens, such as angels and pegasi.