[kian]

I had a teacher in high school, specializing in ancient latin and greek, who told me about the musculus --> little mice connection. He was also a very strong Austrian -- to demonstrate the reason, he pulled up his shirt sleeve, made a classic bicep curl motion, and rotated his fist rapidly from inside to outside facing. It was pretty shocking to see the 'little mice' running under the 'covers' of the skin of his arm -- since then, I've never had trouble believing this particular hypothesis.

[arnsholt]

To add on this, musculus is not only an entirely regular diminutive derivation of mouse (mus + -culus) it's even the same pattern as is used for testicle: it's a small witness (testis, see also latinate English words like testify) of manhood.

[dredmorbius]

TIL:

testis (n.) (plural testes), 1704, from Latin testis "testicle," usually regarded as a special application of testis "witness" (see testament), presumably because it "bears witness to male virility" [Barnhart]. Stories that trace the use of the Latin word to some supposed swearing-in ceremony are modern and groundless.

https://www.etymonline.com/word/testis