If you mean to create a pet like how you made your starter pet, you'd need an essence for that species.
There are four different essence types, which can be bought on the trade broker from other users, or from the Diamond shop.
If you hover over the essence item(Or hold and click it on a touch screen), you can read the descriptions of the essences to learn more about them. In the Diamond shop, they're under the essences section.

You can also buy pets from other users by either looking through the Pet Marketplace forum or through the Advanced Search:
https://www.sylestia.com/search/
https://www.sylestia.com/forums/?forum=3

Some users also do pet raffles and giveaways here:
https://www.sylestia.com/forums/?forum=16

I hope this helps!

Thank you for your help