A very long time ago when I used to regularly make fakemon I often got the question as to how I came up with them. At the time I didn’t really have a good coherent answer nor the English vocabulary to actually elaborate how so I decided to do something special and create a design notes image submission to go alongside the actual starters.
So yeah the first thing I do when I come with any creature design is basically creating a synopsis in my head for it. For these Pokemon it was: starters Pokemon you could find in the Canadian province of Prince Edward Island. Starter Pokemon usually a grass type based on an extinct animal, a fire type based on one of the signs of the Chinese Zodiac and a water type based on a amphibious animal.
From there I kinda brainstorm what could fit those niches and came up with Dimetrodon, Holstein Friesian cattle and a North American beaver. Furthermore most Pokemon are animal with a twist so in this case I tried to apply their elemental types into the designs themselves. Frondon and Beablet have that elemental type around the unique feature of their species though Taurch was a based on the fact cows use their tail to swat away bugs (therefore bug types, which are weak to fire, would be afraid of a flaming tail to be sure). In addition Taurch also is based on a minotaur in addition to being based on a type of domestic cattle which explains some of its design choices. Also I gave Frondon some mammal like features actual dimetrodons likely didn’t have (like erect ears and a deer like nose) to make the fact its based on a synapsid more obvious as I had some concerns people would think its a stegosaurus lol.
From there I put their general shape (often called a silhouette) onto paper and tweaked the designs until they felt like Pokemon. That’s definitely a thing you can’t teach easy but its mostly just learning to balance markings. Beablet is about as complex as you want to push it.
As a last point don’t be afraid to based a fakemon on a creature that already has a Pokemon based on it. As fun as it is to try to come up with designs based on obscure creatures that haven’t got a official design based on them yet that can lead to you trying to force the design. That can definitely lead to designs that just look off. The world will not mourn too heavily seeing another cat Pokemon I promise you, if that’s what your mind wants to make go for it!