Giant Bat

A friendly giant bat is a vampire's lent steed, a cave-druid's wild shape, or a familiar from a rare ritual. The most common framing is mount: the bat is large, the party is small, and somebody negotiated transport. At CR 1/4 it can carry one Small or Medium rider clear, though the rules don't formalize the load.

Use the bat as the party's air access at low levels. Fly 60 ft. is fast enough to outrun most ground threats, and Blindsight 120 ft. means it can navigate caves and tunnels in the pitch dark while the rider holds on and tries not to vomit. It cannot fight while carrying. The rider has to dismount or the bat has to drop them, and the bat is AC 13 with 22 HP, so it does not survive sustained ground combat. Treat it as transport that flees the moment a real fight starts.

The texture is in the squeamishness. Bats are not pleasant. They smell, they screech, they cling. Make the players negotiate for it every time they want to use it. Whoever lent the bat wants it back unharmed, and the bat does not love being a taxi. When the party is done with it, have it climb the nearest ceiling and hang there until called.

A giant bat is a cave-system encounter that uses verticality. The stat block is unimpressive on paper, 22 HP and AC 13 with no actions block, but the speeds and senses are the whole point. Walk 10 ft. and fly 60 ft. means the bat lives in the air and only ever touches ground to die. Blindsight 120 ft. means torches and light spells don't matter. The party is being hunted in the dark by something that can see them perfectly while they squint.

Open from above. The bat is hanging from the ceiling of a 40-foot chamber when the party walks in, and it drops on initiative. With no Multiattack and no special actions, you'll be improvising the bite using the size and Strength to justify a basic unarmed strike, or running it as a slam-and-grapple at the GM's discretion. The honest play is to use the bat as a positional threat: it dives, harasses, knocks over a torch, and pulls back into the dark before the fighter can close. Make the players feel like they're shooting at shadows.

Pairs and small flights are the right call at CR 1/4. A single bat dies to one solid arrow shot, but four bats taking turns dive-bombing the cleric while the rest of the party can't reach them is a real problem. If the party has no ranged options, the bats just keep doing it until somebody runs out of HP. That should never happen by accident, so make sure the encounter has a way to engage them: stalactites the party can climb, a pit they can lure the bats into, a colony roost the party can threaten to scare them off.

Use the bat as ambient horror, not as the boss. A flight overhead with no fight at all sets the tone for the deeper cave better than any monster manual line.