This situation isn't like The Andromeda Strain.
Not that what they did in that film was unjustified, but they actually took healthy animals and exposed them to a space disease, in order to understand its pathology and find a cure.
It's true that they took advantage of already having the split animal to test the procedure for Kirk, but all they did was try to cure a space dog that needed curing, and they didn't sacrifice an otherwise viable animal just for Kirk's benefit. That's not even animal testing; it's experimental exoveterinary medicine
Yes, I don't believe that they'd just randomly snatch space dogs, especially not as isolated specimens. That's a strike against the episode right there. The only reasons the space dog was in the episode were to reveal to the rest of the crew what was going on to Kirk and to amplify the jeopardy that Kirk was in.