long
Explanation
This comic uses the classic "a horse walks into a bar" joke format but subverts it entirely. The setup reads: "A horse walks into a bar. The bartender says: 'Why the long face?'"
The horse responds not with a typical punchline but with an elaborate, overly detailed anatomical explanation: referencing its "elongated nasal cavity," "extended maxillary and premaxillary bones," "masseter and jaw muscles," and other anatomical features, essentially giving a scientifically accurate explanation of why horses have long faces.
In the final panels, the bartender stands in a dark doorway looking disturbed, and the horse says something like "Want 2 mean to" (or similar), suggesting the horse's literal, overly-detailed response has killed the mood entirely.
The joke is an anti-joke or meta-joke: it takes one of the most well-known bar jokes in English (where "why the long face?" is a play on words meaning both "why do you look sad?" and literally "why is your face elongated?") and has the horse interpret it completely literally, giving a zoologically accurate answer. The humor comes from the collision between the expected comedic format and the aggressively literal response. This is a common SMBC technique -- taking familiar joke structures and deconstructing them by having characters respond with unexpected literalism or over-analysis. The bartender's dismayed reaction in the dark represents the death of humor at the hands of pedantic literalism.