If you're getting something small and furry, like a rat or rabbit or hamster, be sure to get two and give them some living space. If you're not able to give them a lot of time, at least let them socialize with their own kind.
We had one rabbit, then two, and the difference in the original rabbit's (OR) personality after getting the second was unbelievable.
Please be careful when introducing two hamsters together. I have had them kill each other - it was sad and not very pretty. They were even siblings but had been separated then brought back together.
I suppose getting two at the same time that were living together at the pet store would be the best option. Make sure you know their gender first or you'll end up with more hamsters.
We had one rabbit, then two, and the difference in the original rabbit's (OR) personality after getting the second was unbelievable.