After trying riding for the first time, I've made a list of my own. The following areas are mostly just random areas inside the dungeon where you're forced to unmount all of a sudden and it's just an annoyance. They don't look or feel tight.
Here you go:
Area name is: Underdark (Shattered Badlands) Urdlen’s Pass
Its dimensions are: 9 x 9
It is considered an interior and underground
You can not mount in this area.
Area ResRef: urdlenent
Tileset ResRef: ttu01
Area name is: Underdark (Middle) The Burning Shore (Dungeons)
Its dimensions are: 15 x 6
It is considered an interior and underground
You can not mount in this area.
Area ResRef: ud_brnshr_twr002
Tileset ResRef: tde01
Area name is: Underdark (Middle) Deep Glen
Its dimensions are: 11 x 8
It is considered an interior and underground
You can not mount in this area.
Area ResRef: ud_gntkingrge
Tileset ResRef: ttu01
Area name is: Underdark (Middle) Dolgom of the Deep Giants
Its dimensions are: 8 x 12
It is considered an interior and underground
You can not mount in this area.
Area ResRef: id_dpgnt1
Tileset ResRef: tcn01
Area name is: Underdark (Middle) Formian Territory (Gates)
Its dimensions are: 8 x 8
It is considered an interior and underground
You can not mount in this area.
Area ResRef: ud_formian02
Tileset ResRef: tdm01
Area name is: Shadow Plane (Shadow Woods) West
Its dimensions are: 10 x 9
It is considered an interior and underground
You can not mount in this area.
Area ResRef: sh_forest1
Tileset ResRef: ttu01
Area name is: Shadow Plane (Shadow Woods) East
Its dimensions are: 9 x 8
It is considered an interior and underground
You can not mount in this area.
Area ResRef: sh_forest2
Tileset ResRef: ttf01
Area name is: Baator (Minauros) Jangling Hiter (Rendering Plant) Inside
Its dimensions are: 9 x 11
It is considered an interior and underground
You can not mount in this area.
Area ResRef: ba_area017
Tileset ResRef: tsw01
Area name is: Dark Spires (Temple of Auril) Level 2
Its dimensions are: 9 x 9
It is considered an interior and underground
You can not mount in this area.
Area ResRef: area076
Tileset ResRef: tdm01
Area name is: Arelith Forest (Lake Benwick) Cave
Its dimensions are: 8 x 8
It is considered an interior and underground
You can not mount in this area.
Area ResRef: ir_lakebenflcave
Tileset ResRef: tmr01
Area name is: Stonehold (Barracks)
Its dimensions are: 9 x 8
It is considered an interior and underground
You can not mount in this area.
Area ResRef: stone_area
Tileset ResRef: tdc01
Area name is: Stonehold (Arcanum)
Its dimensions are: 13 x 5
It is considered an interior and underground
You can not mount in this area.
Area ResRef: stonehold_arcane
Tileset ResRef: tdr01
Area name is: Arelith (Island Ruins) The Descent
Its dimensions are: 12 x 6
It is considered an interior and underground
You can not mount in this area.
Area ResRef: isleruins_desc1
Tileset ResRef: ttu01
The following areas are debatable, but it would be cool if they were added as well.
Area name is: Dwarven Ruins (Library)
Its dimensions are: 10 x 9
It is considered an interior and underground
You can not mount in this area.
Area ResRef: thehallsofknowle
Tileset ResRef: tdr01
Area name is: Underdark (Shattered Badlands) The Crawler’s Burrow
Its dimensions are: 14 x 7
It is considered an interior and underground
You can not mount in this area.
Area ResRef: urdlentunnels
Tileset ResRef: tib01