Theory time...
Warning: contains probably a lot of useless jabbering as this theory is a bit of a reach...but if you like talk of Norse folklore...
When it comes to Lugarugan and his two forms...one when it is bathed in the rays of the sun and the other when full of the night's power...
It is however rather interesting that Rockruff evolves into a wolf of two different forms based on sun and moon...of course it is very possible this could play on how werewolves are alternate forms, but at the same time I feel they may be loosely based off of two specific wolves, who are also associated with the Sun and Moon...
Sköll and Hati from Norse Mythology...
In Norse folklore they are a pair of monstrous wolves who are depicted chasing the chariots that carry Sol and Mani, the Sun and the Moon...During Ragnarok they would eventually catch and devour their prey...
Interpretations and translations vary between which wolf follows which, though it seems Skoll chases Sol (the Sun) and Hati chases Mani (the Moon)...It is also believed that these two are not only connected with Fenrir but are in fact his children...
Now take everything with a grain of salt as everything is a bit of a stretch since this theory is counting on how much Lugarugan is influenced by the legends of Skoll and Hati...as well as Zygarde taking inspiration from the Children of Loki...
As we all know Zygarde has some, rather strange forms...From a monstrous humanoid, to a draconic serpent, to a bunch of little worm-like cells and cores...but Zygarde 10% does indeed stand out rather strangely being very dog-like in appearance...
It is believed that the 10%, 50% and Complete forms are based on the monstrous Children of Loki, with 10% being based on Fenrir specifically...the Green Leash on it's neck may be a reference to Gliepnir which is the binding placed on Fenrir from the gods, not to mention 10% being the weakest of the three may reference that Fenrir is in a weakened state due to his binding...
Now I know what you're thinking 'So Rockruff and Lugarugan are the offspring of Zygarde 10%' and I don't think that's the case...though this theory is moreso based on the premise that Lugarugan's forms may have a possible connection with Zygarde...possibly...maybe a small connection...as said there's not much to go on...
of course finding an excuse to talk about Norse Mythology is always a good excuse for me though I do think it's possible that Lugarugan may be loosely based off of Skoll and Hati if not have a connection to Zygarde...