I made a brief obsevation. In the nāga saṃyutta there are three suttas regarding nāgas doing uposatha. I found this interesting, but noticed that in the following supaṇṇa saṃyutta, gandhabba saṃyutta, and valāhaka saṃyuttas, they are not mentioned observing uposatha.
My question is, are these and other nonhuman beings capable of observing uposatha? What makes nāgas capable of doing so? I find it odd that, for example, the valāhaka devas would not observe uposatha, considering that they are born as a result of good conduct, whereas nāgas are born as a result of ambivalent conduct.
If anyone has the answer to this or any associated information I would be grateful!