In Stellaris?
They kinda are, depending on the path you take.
If you're a psychic power there's an event chain where you can end up making a pact with what amounts to a Great Old One, every one of those Old Ones will cause your populations to have random mutations (and one will make a bargain with you for great power in return for ending the universe...), but there's one in particular that's focused on fertility of your population and empire, and a pact with it will boost your species fertility while spreading a mutation through the population.
On the flip side, if you're a bioengineering power, you can basically MAKE your people into monsters via direct genetic engineering.
So... they'd not really be all their weird in Stellaris?
