Insect Bite Reaction: This is the most likely diagnosis given the history of a horsefly bite. The reaction can cause localized swelling, redness, and puffiness around the bite area, including the right inner canthus if the bite was nearby.

Cellulitis: An infection of the skin and subcutaneous tissues that can occur after an insect bite. Symptoms include redness, swelling, warmth, and tenderness, which could explain the puffiness around the inner canthus.
Allergic Contact Dermatitis: An allergic reaction to substances in the horsefly's saliva can cause dermatitis, leading to swelling, redness, and itching in the affected area.
Angioedema: A severe allergic reaction that can cause rapid swelling of the skin and mucous membranes, potentially affecting the area around the eye, including the inner canthus.

Orbital Cellulitis: A serious infection of the tissues surrounding the eye, which can be life-threatening if not treated promptly. Symptoms can include swelling, redness, and pain around the eye, including the inner canthus.
Cavernous Sinus Thrombosis: A rare but potentially fatal condition that can occur if an infection spreads to the cavernous sinus. Symptoms can include swelling and pain around the eye, among other neurological signs.

Erythema Multiforme: A skin condition characterized by target lesions, which can be triggered by insect bites among other causes. It's less likely but could explain skin reactions and swelling.
Mastocytosis: A condition where there's an accumulation of mast cells in one or more organs, which can lead to allergic reactions, including swelling, after insect bites. However, this would be an unusual presentation.