Its Japanese species name is literally "fire mouse," but it's not right to look at it so literally. That's why we have mistranslations like Weedle being a "hairy bug," Zangoose being a "cat ferret," and Purugly being a "tiger cat."
In Weedle's case, one word for caterpillar literally translates to "hair bug." For Zangoose, the word for mongoose literally translates to "cat ferret," and for Purugly, the word for tabby cat literally translates to "tiger cat."
So, let's look at Cyndaquil. It's pretty obviously a fire porcupine or hedgehog, right? The Japanese word for hedgehog is literally "needle mouse." There wasn't enough space to fit "fire hedgehog," so hedgehog was abbreviated as "mouse."
Sandshrew got a similar treatment; the word for armadillo, which is literally "armor mouse" would not fit, so it was also abbreviated to just "mouse."
That's my take on it, anyway. It makes much more sense to me than some Chinese zodiac nonsense.
SERIOUSLY NOT HOME ROWED want some funeral to go with that cake?