I think Bill "Grumpy" Jenkins may have started the 'mouse' name with his potent small block Chevy's in NHRA racing when he put the hurt on Mopar's "elephant" hemi's. From there the Mark IV big block Chevy came out and followed suit with 'rat motor'. According to an article on Gasser Madness, a guy named Skip Hess is given credit for coining the term "rat motor" when the first AHRA racers started dropping blown big blocks in Anglia gassers in 1965. It was painted on the Shores and Hess Anglia scoop first:
Small block Chevy V-8 was refered to being a mouse because of it's power to weight ratio starting about 1956-'57. The only engine with a better ratio is the 455 Buick engine which is about 30 pounds heavier than a SBC but about the same size (it wasn't introduced untill 1970). It was the previously unheard of lightness of the engine that caused everyyone to drop the other 1957 power champ; the 364 cid "Nail Head" Buick motor, and and the older 1947 vintage 331 cid Caddy motor that weighed a ton compared the SBC that was out putting the same amount of power out of a lighter package. In racing every 100 pounds is one tenth of a second off your elapsed time so a smaller more pottent power plant became very popular very quickly.
The "Rat" was the "Mouse's" distant cousin which could be made to deliver lots of power but was never considered light weight, and to this day still can not match the horsepower per cubic inch of the diminutive mouse motor.
I have been breeding and feeding Rat motors since they were introduced. Always looking to give a Rat a good home.