By sterile I mean unable to breed since when you look at sunny and even Darkstalker and the dragon in burns stronghold especially it's obvious that while its possible DNA from different dragons is not something nature approves off. For instance the dragon in burns strong hold barely had any wings and sunny can't properly blend into the sand properly and she has no tail barb as for Darkstalker we don't know much about him he has animus and nightwing powers but I don't think he can breath fire the only reason he has Nightwing powers is because he probably inherited more of his mums genes and he hatched on night of the three moons. But I don't think he can breath fire or frsotbreath for that mater since I feel like two genes would almost cancel eachother out in a way, Since most hybrids in our world there genes are mixed up and unnatural look at ligers tiger and lion hybrid while they are big they are too big if they were to be released in the wild they would end up starving not to mention the fact that they can't breed and are prone to sickness. Mules also can't breed either so I'm on the side that hybrids can't breed and re just plain out sterile since that's nature way of preventing things like this.
On the other hand they could just end up like dogs but I think dragon DNA is not like dogs or cats the reason why dogs of different breeds can produce a fertile offspring since there both dogs wolves and dogs can breed and produce fertile offspring is because there both subspecies of Cains lupus. However horses and donkeys can breed but they have sterile offspring this is because while horses and donkeys have similar chromosomes to mate but there two different to create a fertile offspring explaining this without confusing people is hard but I'm sure most of you are getting the gist of it. While I'm not to sure on how the genetics of wings of fire dragons are from what I've seen in the books it feels like there DNA is not meant to mix and when it does it usually doesn't mix well by natures laws its "unnatural" But for all we know they could just be like dogs and be subspecies instead of species .
I'm not an expert on genetics I really only have what they teach you grade 9/10 I did do some research in this as genetics are very interesting to me. But I'm also here to talk about why I dont think stonemover and sunny are related to Darkstalker while its cool to think. If we are going with the theory with hybrids being sterile than that means None of Darkstalkers siblings could have offspring which means he couldn't be able to as well. So why are there nightwing animus dragons than? Well how do you think Animus dragons came into at existence first? I feel like the animus gene is a mutation (Please google gene mutation as if I were to explain it it would take up a large portion of this blog) but I think it might have been a gene mutation and obviously other dragons with that mutation might have bread with eachother and thus animus dragons are born while its has been stated that only one parent needs to be animus for offspring to be produced its less likely to happen with only one animus parent however that wont mean the offspring wont have the gene hidden in there DNA, so if they were to have an offspring there is a possibility of that offspring having animus powers. Now I have two theories here Either Stonemovers parents had a dormant animus gene in them and Stonemover happened to be born with animus powers that way or all dragons have dormant animus genes in them and its so rare for it's actual traits to be passed down onto an offspring that they just concluded that only animus dragons can pass on that gene since its more common to happen. But I'm just speculating here none of this us probably true and trust me this genetic stuff is probably not accurate its been 1 year and half since i last did genetic stuff so yea. But I'd love to hear your thoughts on this subject.