Which muscle is responsible (weak or non-functional side)?

Right lateral pterygoid The lateral pterygoid is the prime mover for mouth opening and protrusion. Contraction of the RIGHT lateral pterygoid pulls the right condyle forward and medially, which deflects the mandible to the LEFT. If the mandible deviates to the left on opening, the right (contralateral) lateral pterygoid is the culprit — it is overactive or hyperfunctioning. Rule: "The side that deviates is the AFFECTED/weak side; the responsible muscle is CONTRALATERAL."