My first virtual pet ever was a nano kitty. I got quite a few of them in my collection.
On ebay for one of the newer looking ones that are sort of square, they're quite reasonable in price. They come in colours of Gold/Orange, Green and Pink/Red.
The older made ones that are oval will be harder to find at a decent price. They can come in white/silver, green, purple and orange.
The main difference between the newer ones and older school ones are just the outside. The programming is all the same
In the beginning there was just nano kitty, nano puppy and nano baby, but I know there are other kinds out there like the nano fighters, nano double where you care for 2 pets and other odder ones like nano bunny and nano dino.
The neediness on these guys aren't bad at all. You can't pause them but you can alter their time like you would a tamagotchi to make it sleep and wake up. It goes to bed at 9pm and wakes up at 7am all the time so it's not a hard schedule to get used to. It also will nap at various times, sometimes in a pattern.
I think the only main different between the nano baby and the animal versions is it ages 1/3's of a year everyday and only stays until it's 3 years old, then you raise it's younger sibling. The kitties and puppies age one year everyday and can live quite a while. I had my one green nano kitty I first got to start off my collection it lived to be 36 years old, which is pretty good
All of them will have a number of times it'll grow bigger and the growth is always the same. Not much change. Watching them interacting in their rooms though is pretty cool
Overall I enjoyed them as a non-bandai pet and they are worth the try. If you can too, get a hold of the talking nanos if at all possible. They add a whole different play to the nanos with a face talking at you