After Mexico gained its independence from Spain, the newly independent country organized itself into several states. In the northern Coahuila y Tejas, there were many Native peoples like the Alabama, Apache, Aranama, Atakapa, Caddo, Comanche, Coahuiltecan, Cherokee, Choctaw, Coushatta, Hasinai, Jumano, Karankawa, Kickapoo, Kiowa and Wichita that the nearly bankrupt Mexican government had little resources to subjugate. So they invited immigrants from the US, called Texians, to help keep down the aborigines.
Soon the immigrants outnumbered the Mexicans and Natives put together. These Texian immigrants made little to no effort to assimilate into their adopted country -- they they self-segregated, carried guns everywhere, didn't learn "the language" (Spanish) and wrote signs in English. Even though slavery was illegal in Mexico, the Texians (who numbered about 30,000) simply ignored Mexican law and brought 5,000 slaves. Before long, Mexican president Bustamante sought to restrict futher American immigration to Mexico, recognizing they were up to no good. Before long, the Texians took up arms and ultimately gained independence from Mexico.
Joel McCrea, not Texian, but played one on the radio
By 1850, Texians started referring to themselves most commonly as Texans. The Texas Almanac of 1857 waxed purple about the mere dropping of the letter "i," continuing the Texan tradition of making something out of nothing, moaning [in Chris Elliot's fancy lad voice] "Texian...has more euphony, and is better adapted to the conscience of poets who shall hereafter celebrate our deeds in sonorous strains than the harsh, abrupt, ungainly, appellation Texan -- impossible to rhyme with anything but the merest doggerel."