The Collaborative International Dictionary of English v.0.48:

Dakotas \Da*ko"tas\, n. pl.; sing. Dacota. (Ethnol.)
   An extensive race or stock of Indians, including many tribes,
   mostly dwelling west of the Mississippi River; -- also, in
   part, called Sioux. [Written also Dacotahs.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dacotahs \Da*co"tahs\, n. pl.; sing. Dacotan. (Ethnol.)
   Same as Dacotas. --Longfellow.
   [1913 Webster]