Pigs do not pant or sweat and need water to cool off in
all i could find on origination of this is below:
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The meaning, and origin of the term "sweats like a pig":
From what I have been able to gather the origin of the expression `sweat like a pig,' meaning to sweat profusely, is unknown. Since pigs don't have sweat glands, which would provide cooling by evaporation, they have to wallow in puddles and mud to cool off. For an appropriate simile `sweat like a horse' might be more accurate. Speculation is that the idea that pigs sweat profusely could have derived from the fact that they smell, with the assumed cause being `sweat.' The only information in any respectable sources that I could find was that the expression, along with `sweat like a bull' (and bulls do sweat), dates from the late 19th century (Cassell's Dictionary of Slang). In Palmatier's `Speaking of Animals - A Dictionary of Animals Metaphors," it is suggested that the `like a pig' may have evolved from the model of the older expression `bleed like a stuck pig' (17th century) - but that sounds a bit weak. Some other `pig sweating' expressions according to Palmatier are `pig sweat' meaning beer (however, he may be one of the few people in the world that has heard of that one) and `sweat hog' (1970s and still in use) meaning 1) a difficult student singled out for special attention (the expression may have been coined in the TV series `Welcome Back Kotter') 2) an unattractive female 3) a promiscuous female.
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