"He has beautiful dogs."
Translation:Il a de beaux chiens.
"Des" becomes "de" if it is immediately followed by an adjective, in this case, "beaux."
Why not 8de belles chiennes*? Because French defaults to the masculine if no grammatical gender is specified. English does the same. When we talk about a group of dogs, we mean animals of both sexes. Even if they are all female, we do not say "bitches" unless the sex is important to the discussion - as it might be for vets, for example.
On a wider point, other Indo-European languages do the same, and the origins may well be in the masculine bias we see in language development; but apart from legislation and protracted argument, I don't see how things can change. I would like to get rid of gender-based words where feasible. English does not need terms like comedienne or murderess any more than Irish needs banaltra (specifically female nurse) or Dutch ober (waiter), *serveester (waitress). But I suspect we'll have a long wait.
Not sure if this is an error, but the feminine form for dogs is not being accepted.