You’re right. There are a variety of slang names for vaginas that don’t seem to make sense. They are often referred to as beavers, pussies, twats, cookies, and other unlikely names. It does make you wonder.
There actually is a fairly reasonable explanation for labeling a woman’s lady parts as a beaver. Back around the 17th century people came to believe that sexually transmitted diseases and crabs were passed through contact with pubic hair so prostitutes were ordered to shave off their pelvic curls. That’s when bordellos earned the moniker baldy houses. The hookers’ customers, however, found the bare look unnatural and unattractive and the women were losing business. Their solution? To have little wigs known as merhkins made for their crotch. The material of choice for these hairpieces was beaver pelts, selected because of the natural feel of the beaver’s hair. Since that day vaginas have often been referred to as beavers.