It is quite simple: Mix 2 monsters to 1 stronger monster!
Monster from the same generation can be mixed to a stronger one. The mixing process will result in 1 monster which will get the skills from its ancestors. Mixing monsters is essential to make some room for new stronger monsters in your hideout and to obtain monsters of new generations faster.
After mixing monsters, always make sure you summon a fresh baby monster to fill up the empty gang slot.
By mixing monsters you can create strong beasts with all skills and the maximum level!
Once you click on a monster, you can activate the respective button for mixing, to see which monsters are available to mix your current creature with.
Mix it, baby!