how is it possible as a Druid to resist the spell “scare beast” while in beast-form?
Be lucky on a spell “miss”
Get an immunity from a priest
Shift out of form
what is the probability?
It will depend on the the casters hit chance and if/any level difference between the two. Just drop form as soon as you see it cast, most bad druids forget they have the spell let alone use it.

how is it possible as a Druid to resist the spell “scare beast” while in beast-form?
u should study the stats weights. hit, miss, dodge, parry and resist.
In PvP baseline you have 4% to have your spells resisted (miss, which writes as resist). If you get spell hit rating you can reduce it to 1%.
You always have 1% chance to have your spell resisted no matter what.
Is this the first time you ever see a spell getting resisted?

most bad druids forget they have the spell let alone use it.
I’m the moron who will shift out but insta shift back in and eat the fear/sleep.
Scare beast is a hunter spell though, we have hibernate (sleep beast).