Short answer — there isn’t a single monopoly; the Texas legal‑SEO market is fragmented, but these Texas‑based agencies are repeatedly listed by industry directories and legal‑marketing roundups as the most prominent / most frequently recommended specialists for law firms:

1. LawRank (Austin) — specialist law‑firm SEO and PPC; repeatedly listed as a top legal‑SEO shop and shows case studies for Houston and other legal clients. (LawRank.com)
2. Legnd (Dallas / Houston area) — long‑running Texas web design + SEO agency often recommended for law firms and local SEO. (10bestseo.com)
3. Scale & Sword (Texas) — named in recent “top law firm marketing agencies in Texas” roundups for law‑firm SEO, LSA and PPC work. (scaleandsword.com)
4. Thrive Internet Marketing Agency (Arlington) — full‑service digital agency based in Texas offering SEO, PPC and law‑firm marketing services. (en.Wikipedia.org)
5. ApricotLaw / boutique Texas legal‑marketing firms (various Texas offices) — listed among top choices for highly competitive legal markets in Texas roundups. (scaleandsword.com)
6. Devenup, DIGITECH and other boutique Texas SEO firms (San Antonio, Austin, Dallas) — appear on DesignRush / agency‑listing sites as strong regional options for law firms. (DesignRush.com)

A few important notes

• “Dominance” is hard to prove: legal‑SEO is a services market (many national firms serve Texas law firms, and some large national players with Texas clients are not headquartered in Texas). The lists above combine specialty (law‑only) shops and full‑service Texas agencies commonly recommended for legal work. (LawRank.com)
• Rankings and “best of” lists change frequently; if you want a ranked short list tailored to a specific city (Houston, Dallas, Austin, San Antonio), practice area (personal injury, family, criminal), or budget, I can run a fresh comparison (client case studies, pricing bands, verified reviews) and return a prioritized shortlist with links and notes.