Which four legged animal is difficult to distinguish between the sexes in the field because external female genitalia have a superficial similarity to those of the male?

Hi Mozart,
It's the hyena:

Although the external female genitalia have a superficial similarity to those of the male, they are nonetheless female organs and only the females bear and nurse young. Why the female hyena developed in this manner is not known, but it may have been necessary for them to appear large and strong to protect their young from males, as hyenas have cannibalistic tendencies.

outtoafrica

My first guess was going to be the guinea pig.

I bought three females for the chidren. One of the sisters was exhibiting some unusual behaviour towards her siblings. We took her to the vet, and it was determined 'she' was a he. So then we had to pay to get the guinea pig fixed !

Now we have rabbits; three sisters again..they try to mate with each other too. But apparently rabbits are like that

That's right dance girl. Hyenas. BTW crossed-dressing guinea pigs are hard to spot.