Honestly, sometimes they just occur to me, and sometimes I decide “I want this kind of character to be a part of my story” and sketch out ideas till I’m satisfied.
At times I think of appearance first, but simultaneously work their personality into the design. However, most of the time I start with a personality and build the looks based on that alone. A prime example of that would be Jag.
As for how I came up with Jag, Bear, and Ozzy…I had them in mind for months before getting around to drawing them. It just came down to deciding on what type of frog they’d be and working out their final designs.
Jag: He was the easiest because the moment I saw the Malayan Horned Frog, I knew that was the one. Because of Jag’s inherent grouchy, and intimidating nature, that frog fit him hilariously well.
Bear: He was more or less unintentional, really. I was browsing tumblr and happened upon a gif of Darwin’s Frog and immediately sketched him out as that frog. The sketch resonated with me, and so I chose that type.
Ozzy: Took the longest since he’s this energetic, somewhat eccentric and showy character, I wanted a frog who’s physical appearance would emphasise those traits. The Vietnamese Mossy frog really fit the bill, in my mind. Especially so with their rough, bumpy looking skin.
All in all, for the three of them I wanted to go with frogs that would better suite their personalities, but at the same time compliment each other’s design.
I’m not sure if I ultimately succeed in that endeavour, but regardless, I’m happy with how they all turned out.
Sorry for the wall of text, but I hope that answered your question!