Breezy
From Memory Alpha, the free Star Trek reference
(written from a Production point of view)
Breezy was the beagle that worked as back-up and then took over the role of Porthos after the episode "Two Days and Two Nights". Breezy was a bit more "spunky" than Prada, so, when they needed a more playful dog during the first season, Breezy was used—most famously, in the "where no dog has gone before" shuttlepod exit on location outdoors in "Strange New World". After the first season, Breezy took over the role full-time.
Although Porthos was a male dog, as was Prada, the series' producers didn't want the audience's attention drawn to Prada's "male components", because the idea was to replace him with Breezy, a female, and hope the audience wouldn't notice the change. They also had to paint a temporary black spot on Breezy's side during shooting in order to match Prada's color.
