I'm thinking on developing my own breeding game. Should be easy enougth to create the basic game mechanics and mostly the planning phase is done. I'll use "NimbleBit"s "Pocket Frogs" (https://en.wikipedia.org/wiki/Pocket_Frogs and https://play.google.com/store/apps/details?id=com.nimblebit.pocketfrogs&hl=de) as example, because my idea is in some ways quite similar. Android is the first platform I want to target, with optional expanding later to IOS and perhaps Windows.

First Problem:
I'm quite not sure about what language or engine I should use. I know this question is mainly based on opinion, but based on what I plan to do, is there an engine to prefer and why, or should I build it from scratch and wich language should I use then? At the moment my best bet would be on unity or python. Any suggestions here?

Second (and most significant) Problem:
I want to create a large amount  of - let's call it - monsters, that differs in color, pattern, color of the pattern and partly shape, but I don't get the trick behind it. So the question here is: How to create reusable monsters that differ in the above mentioned characteristics, with the lowest possible number of graphics.

My thoughts and attempts on that topic: I looked at Pocket Frogs, and except shape they do exact the same with their frogs what I want to do. But I really don't get how they created over 38.000 (!!!) individual frogs, and the game still doesn't use that much space. I first tried to extract the graphics from the games files to puzzle together what they did. But I could not find them. However I think I figured out some parts of this secret just by looking at the frogs ingame: I think they used a basic frog model. 16 to be exact, to create 16 background frogs in all the colors. On top of them they just displayed the different patterns. But - and thats the mystery - the patterns are in different colors too and  I still dont believe they made 16*104=1664 different pattern graphics. So what trick am I missing here? Some kind of mask? Can I use the same technique to create different additional shapes for my monsters? And how did they made the feet moving. If the pattern on the feet are extra graphics, that would be another 1664 graphics.

Any idea on how I can make this work, or on how did they make this work will be very appreciated! Thx