Cryptids

[Mister Agenda]

Bigfoot/Yeti -<1%.
Aliens- I'll say 80%. Lots of planets, but we don't know the odds of life beginning. I have trouble concluding that it's vanishingly rare, though because billions of planets.
Intelligent Aliens (ones that at the very least have our level of comprehension) - 1%. No guarantee that life will turn out the way it did here; I think our double planet arrangement led to land life developing much sooner than it would on a planet with smaller tide variation.
Aliens that have visited Earth - <1%
The Loch Ness Monster - 0%
Chupacabra - <1%
The Jersey Devil - 0%
Mokele-mbembe - <1%

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Picture our galaxy as a petri dish.

A nice analogy but I'm not sure just how relevant it is.

The galaxy is much more like a giant room filled with millions of petri dishes all spaced miles away from each other. Spreading on one petri dish isn't all that inconceivable, spreading to every other petri dish would take something extraordinary, in fact, we have no real way of knowing how likely or unlikely it is, because as far as we know, it hasn't happened yet, and it isn't even necessarily possible.

Consider these points:
1. We are the only petri dish in the room that has formed life, every other dish we know of doesn't have such life.
2. Life has existed on the petri dish we call Earth for 4.6 billion years. The universe is 13.8 billion years old.
4. In such a significant portion of the age of the universe it has taken our example (the only example) of life quite a while to show any indication of moving away from its starting point.
5. The furthest life on this petri dish has ever got is the moon, which isn't even a babystep considering the size of the galaxy.

Now, it's highly plausible that once one civilisation gains this spreading ability it could go on and spread to every other petri dish. But, we have no way of knowing whether this spreading ability you talk about, is even remotely likely in the first place. Considering the time it has taken life on this petri dish to form, let alone evolve and then even get close to venturing away, I'd say it takes a lot longer for life to spread than you suggest.

Saying something like "give life 10,000 bites at the apple, the first one to develop the spreading ability wins the prize" is just trivial, because it says nothing about the possibility or plausibility of said spreading ability.

Once upon a time I calculated that with a travel capability of 10% of lightspeed, it would take us about 2 million years to colonize the galaxy end-to-end. We don't have to break the laws of physics to occupy the galaxy, but it does presume the will to do so.