What do you mean? The difference is in definiteness, if you use the as in the rabbit, you have a specific rabbit in mind. When you use a, you have a general idea of rabbits in mind.
If you ask how to render that difference in Gaelic – you use the word without any article for the indefinite meaning (coineanach a rabbit), like you would in English plural (eg. rabbits aren’t big, not
a rabbits or anything like that), and for the definite one you use the definite article an/a’/am/na (an coineanach the rabbit).
So to say the rabbit is not big you’d need chan eil an coineanach mòr.