Protection from everything means a permanent cannot be targeted by, enchanted/equipped/fortified by, blocked by, or have damage dealt to it by sources with the relevant quality — in this case, any source at all (CR 702.16b, 702.16j). Damage prevention is one of the four things protection does, often remembered by the acronym DEBT (Damage, Enchant/Equip, Block, Target).
When a fight spell (such as Prey Upon) resolves, each creature deals damage equal to its power to the other creature (CR 701.12a). This damage is still "damage being dealt" and is therefore subject to prevention effects, including protection.
If a creature has protection from everything, any damage that would be dealt to it — including damage from a creature it is fighting — is prevented entirely. Note that the creature WITH protection still deals its damage normally to the opponent; protection only shields the protected creature itself.
Example: Your opponent's 3/3 creature fights your creature that has protection from everything (e.g., a creature with Progenitus's ability). The opponent's 3/3 would deal 3 damage to your creature, but protection prevents all of it. Your creature's damage to the 3/3 still resolves normally.
Important caveat: the fight spell itself could still be cast targeting your creature if protection from everything was granted after the spell was cast. If protection was already in place when the spell was cast, the spell would be countered for having an illegal target (CR 608.2b). Either way, the protected creature takes no damage.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.