King under the Mountain

"… they grew immensely rich and famous, and [Thrór] was King under the Mountain again, and treated with great reverence by the mortal men who lived to the South, and were gradually spreading up the Running River as far as the valley overshadowed by the Mountain."

—Thorin relating the outcome of his grandfather's return to Erebor

The King under the Mountain, also called the King of Erebor, was the hereditary title of the Dwarven ruler who resides at the Lonely Mountain. The King under the Mountain was also by right the King of Durin's Folk.

In The Hobbit, the Dragon Smaug was held to be the only "king" of Erebor. Thorin Oakenshield was in fact in line to be the only rightful King under the Mountain.

Kings under the Mountain

Name	Years	Information
Thráin I	TA 1999 - TA 2190	He founded the first kingdom in Lonely Mountain and discovered the Arkenstone.
Thorin I	TA 2190 - TA 2210	The son of Thráin I he abandoned Lonely Mountain in TA 2210.
Abandoned	TA 2210 - TA 2590	The Arkenstone was left when the mountain was abandoned.
Thrór	TA 2590 - TA 2790	He returned to the Lonely Mountain following the death of his father Dáin I in TA 2589, he rediscovered the Arkenstone and refounded the Kingdom. In TA 2770 with the coming of Smaug, King Thrór was forced into exile until he was later murdered by Azog in TA 2790.
Smaug	TA 2770 - TA 2941	In TA 2770 Smaug conquered the Lonely Mountain and forced King Thrór and the surviving Dwarves into exile and became the de facto king until his death.
Thráin II	TA 2790 - TA 2850	The son of Thrór he succeeded his father in TA 2790 as de jure king, and died a prisoner in Dol Guldur in TA 2850.
Thorin II Oakenshield	TA 2850 - TA 2941	The son of Thráin II he succeeded his father in TA 2850 as de jure king. Thorin II Oakenshield was briefly restored to the throne in TA 2941 after the death of Smaug. He was killed shortly afterward in the Battle of the Five Armies along with his nephews Fíli and Kíli.
Dáin II Ironfoot	TA 2941 - TA 3019	The son of Náin and second cousin of Thorin II Oakenshield, Dáin II Ironfoot succeeded Thorin II in TA 2941. He died during the Battle of Dale in TA 3019.
Thorin III Stonehelm	TA 3019 - FO 137	The son of Dáin II Ironfoot he succeeded his father during the siege of the Lonely Mountain and ruled in the aftermath of the War of the Ring.
Durin VII	FO 137 - FO "unknown"	The son of Thorin III Stonehelm. He succeeded his father after his father's death in FO 137.

Translations

Foreign Language	Translated name
Afrikaans	Koning onder die Berg
Albanian	Mbreti në këmbët e Malit
Amharic	ተራራ በታች ንጉሥ
Arabic	الملك تحت الجبل
Armenian	թագավորը տակ լեռան վրա
Azerbaijani	Dağ altında Kral
Belarusian Cyrillic	кароль пад гарой
Bengali	পাহাড়ের নীচে রাজা
Bosnian	Kralj pod Planinom
Bulgarian Cyrillic	цар под планината
Cambodian	ស្ដេចនៅក្រោមភ្នំនោះ
Corsican	Re sutta la Muntagna
Croatian	Král pod Horou
Czech	Král pod Horou
Danish	Kongen under Bjerget
Dutch	Koning onder de Berg
Esperanto	Reĝo sub la Monto
Faroese	Kongurin undir fjallinum
Filipino	Hari sa ibaba ng Bundok
Finnish	Vuorenalainen Kuningas
French	Roi sous la Montagne
Frisian	Kening ûnder de Berch
Galician	Rei debaixo da Montaña
Georgian	მეფე მთის ქვეშ
German	König unter dem Berg
Greek	Βασιλιάς κάτω από το Βουνό
Gujarati	પર્વત હેઠળ રાજા
Hebrew	מלך בטן ההר
Hindi	पहाड़ के नीचे राजा
Hungarian	Hegymély Királya
Icelandic	Konungur undir Fjallinu
Indonesian	Raja di bawah Gunung
Irish Gaelic	Rí faoi na Sliabh
Italian	Re sotto la Montagna
Japanese	山の下で王
Kannada	ಮೌಂಟೇನ್ ಅಡಿಯಲ್ಲಿ ಕಿಂಗ್
Korean	산의 왕
Kurdish	Padîşah di bin Çiyê (Kurmanji Kurdish)
Latvian	Karalis zem Kalna
Lithuanian	Karalius pagal Kalno
Luxembourgish	Kinnek ënnert dem Bierg
Macedonian Cyrillic	цар под планината
Maori	Kingi i raro i te Maunga
Malagasy	Mpanjaka eo ambanin'ny trano tendrombohitra
Malaysian	Raja di bawah Gunung
Maltese	Re taħt il-Muntanji
Mongolian Cyrillic	Уулын дор хаан
Nepalese	पर्वतमा अन्तर्गत राजा
Norwegian	Kongen under fjellet
Pashto	د غره لاندې پاچا
Persian	پادشاه در زیر کوه
Polish	Król pod Górą
Portuguese	Rei Sob a Montanha
Punjabi	ਪਰਬਤ ਦੇ ਹੇਠ ਪਾਤਸ਼ਾਹ ਨੇ
Romanian	Rege de sub Munte
Russian	Король под Горой
Sindhi	جبل جي ھيٺان بادشاهه
Scottish Gaelic	Righ fo Bheinn
Slovak	Kráľ pod Horou
Slovenian	Kralj pod Goro
Somalian	Boqor buurta Hoosteedii
Spanish	Rey bajo la Montaña
Swedish	Kung under Berget
Tajik Cyrillic	Подшоҳ зери кӯҳ
Tamil	மலை கீழ் ராஜா
Telugu	మౌంటైన్ క్రింద కింగ్
Turkish	Dağaltı Kralı
Urdu	پہاڑ كے نیچے شاحانشااح or شاحانشااح پہاڑ (poetic)
Ukrainian Cyrillic	король під горою
Vietnamese	Vua dưới Núi
Welsh	Brenin o dan y Mynydd
Yiddish	מלך אונטער דעם באַרג