The Collaborative International Dictionary of English v.0.48:

Mulish \Mul"ish\ (m[=u]l"[i^]sh), a.
   Like a mule; sullen; stubborn. -- Mul"ish*ly, adv. --
   Mul"ish*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mulish
    adj 1: unreasonably rigid in the face of argument or entreaty or
           attack [syn: hardheaded, mulish]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "mulish":
   asinine, balking, balky, bigoted, bovid, bovine, bulldogged,
   bulletheaded, bullheaded, caprid, caprine, case-hardened, cowish,
   cowlike, deerlike, dogged, dogmatic, equestrian, equine, fanatic,
   fixed, goatish, goatlike, hardheaded, headstrong, hircine, hoggish,
   hoofed, horsy, intolerant, obstinate, opinionated, overzealous,
   ovine, persevering, pertinacious, perverse, piggish, pigheaded,
   refractory, restive, ruminant, self-willed, set, sheepish,
   sheeplike, stiff-necked, strong-willed, strongheaded, stubborn,
   sulky, sullen, swinish, tenacious, uncooperative, ungovernable,
   ungulate, unregenerate, unruly, willful, wrongheaded