When any living entity falls into the space of a ladder or vine block, their fall distance resets and they 'climb' down the ladder/vine. For certain entities, such as players, endermen and zombies, this is consistent. For entities like pigs and sheep this is not consistent though: real pigs and sheep can't climb ladders, neither up or down.
A more consistent solution would be to separate living entities in those that can climb and those that can not climb:
- Snow Golem
- Wandering Trader
- Zombie Pigman / Zombified Piglin
- Cave Spider
- Iron Golem
- Wither Skeleton
- Zombie Villager
- Skeleton Horse
- Tropical Fish
- Polar Bear
- Elder Guardian
Other entities don't really make sense whether they do it or not.
- Place a column of climbable blocks
- Spawn a pig on top of the column so that the pig falls down
See the pig climbing down: pigs can't even climb...