Hi, this is not something that happens in Buddhism. In my honest
and direct opinion, if supposedly a god 'sucks' the milk, then it
is not a celestial god and definitely not a Bodhisattva. It is the
doing of a Ghost-Deity/Gui Shen, which is a type of ghost who are
powerful and have some good karma, and some people worship
them.
Celestial beings do not drink milk nor eat meat, so if any sort of
beings appear in your dreams and ask you for animal sacrifices or
something like that, it is beyond all doubts that it is definitely
a ghost-deity from the lower realms. Celestial beings have food
that are far better. Their food is not earthly food, it is
completely beyond what we can imagine. Meat and the like are
totally disgusting to them - that's like eating disgusting
corpses.
In Buddhism, we do not worship any ghost-deities and it is not a
good thing to create any such karmic affinity with the lower realm
beings.
Also, please know that temples that worship Guan Yin does not mean
it is a Buddhist temple, there are lots of non-Buddhist folks
temples that worships Guan Yin as well.
As my dharma teacher said before... a lot of those temples that
worship Guan Yin that claims to be able to communicate with Guan
Yin, etc, those are actually ghost-deities communicating with the
medium. They are NOT the real Guan Yin. The above could be a
similar case.
p.s. BTW, even Celestial gods are NOT the same as Bodhisattvas.
Bodhisattvas and Buddhas are enlightened beings, they are much much
higher than ordinary celestial gods. Celestial beings take Buddhas
and Bodhisattvas as their teachers. The difference is that while
celestial beings have very good karma and thus are reborn in the
heaven realms, they still have not eradicated ignorance and are
still under the cycles of samsara -- his temporary enjoyment will
eventually end and he still has to undergo cycles of rebirth. An
enlightened being transcends all samsaric cycles.
