First of all, there is more than one Umbra book, and they are pretty much the companion of Axis Mundi (2nd Ed) on Spirits. These books are pretty much what is given for info about the Garou side of the Umbra. So, let's look at what I have at hand for The Beast.
Axis Mundi (2nd Ed) has nothing to say about realms or this giant "spirit/realm".
Umbra (Revised) (2001) p 107 is in the middle of the "Important Spirits" section, so you hadn't that for sure. But its Realm section, especially on the "other realms" does have no The Beast.
W20 Umbra The Velvet Shadow p 107 is the last page of "Worlds Beyond" subsection "Venturing into the deep Umbra". It contains no info on The Beast.
My deduction skill, and a call at a friend who has it, told me: You have Umbra: The Velvet Shadow (2nd Ed) (1993). The aforementioned paragraph on p107 describes it as:
The Beast is [...] a colossal spirit. [...] Bane Parasites live on the Beast, feeding on its Gnosis. The Beast shifts in the Umbra.
Looking over the edge of the plate, I found Mage - The book of Worlds (1st/2nd ed?) that is looking at the Umbra from the position of the mages. It has lots of places, even quite some that were touched on in Umbra, but none are labeled as The Beast.
The Kindred of the East supplement The 1000 Hells again features new and exciting places to go in the Umbra (especially for Hengeyokai) - and page 39 features a place to go in The hell of Burrowing Maggots that is labeled as The Great Beast. I can't say if this is the same, but it gives a similar (If more creepy) vibe. It is described with about 2 paragraphs, from which I pulled this excerpt:
[...] a formation that appears to be a large white mountain. [...] The outcropping [...] is actually the remains of a creature of immense size that died in this place long ago. [...] The creatures that feed off of it cover it like a swarming blanket
There is also a small box about The Great Beast on p40, hinting origins of it and stating that, while being eaten by the maggots, it moves.
This could be
- the same beast at the same time, ons
part of it being devourd by maggots, the other still kicking.
- the same beast but at different times. One visits a past and future version of it. What is now is to be determined. Time in the Realms is sometimes non-linear at best, a DrWho entangled yarnball at worst.
- a different beast alltogether.