If you're only picking one thing in a probability question, then the actual numbers don't matter - you just need a ratio. For example, if you pick one person from a group, and the ratio of women to men in the group is 2 to 1, then the probability you pick a man is 1/3.
But as soon as you pick two or more things without replacement from a group, then your numbers will matter. Again, if the ratio of women to men is 2 to 1, and you pick two people without replacement, and want to know the probability both are men, then the answer could be zero, if we only have 3 people in total (because then there aren't two men in the group at all that we could even pick), but will not be zero if we have a larger number of people. In this situation, the probability the first selection is a man is 1/3, and the probability the second selection is then also a man will be less than 1/3 (since we've reduced the proportion of men in the group with our first selection), but will get closer and closer to 1/3 the larger the group is.
If instead you make selections with replacement (so your first selection goes back into the original group before you make the next selection) then again, only the ratio matters, since, using the example above, the probability of picking a man would be 1/3 each time.
No real GMAT probability question would be worded the way the question above is, incidentally. A probability question needs to make clear how many selections you're making, and how you're doing it, and this question doesn't even mention that you're selecting exactly two animals.
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