For the animal theme, there isn't exactly a page where we can look up what the Pokemon is based on. SOme are obvious, like most of the dogs, while some are not. The determination of what the POkemon is is going to be left up to you, but it has to be within reason. You can base it off of its appearence, its dex entry, its name (though as Helio pointed out this isn't the best method), or in most cases, Bulbapedia has a page on the POkemon stating where they believe its design originates from.
One thing I wanted to note is in the case of fantasy animals, such as dragons. There are a lot of dragon types out there who frankly don't look like dragons (Altaria, I'm looking at you) If you decide to use dragons, please stick to either the Western shaped variety (4 legs/arms and wings) or the Asian variety (serpentine, like Gyarados). One exception I can think of is Kingdra, who is based off of the Japanese belief that sea horses are baby dragons, hence its dragon typing. If there are any questions in regard to this, don't hesitate to ask me.