If you lower almost any mob's generic.follow_range attribute below 16 (which is the default for most mobs), then their maximum passive wandering range seems to become equal to that number. I believe 16 is the normal max wander range regardless of how high it is, but setting it to something like 5 for example will only ever allow it to move up to 5 blocks at a time whenever it does. It also seems like the lower it is, the less they move at all, presumably because on many of the "wander" checks, the game tries to tell them to move further than their follow range and thus fails. That's just my guess though.

Mobs won't ever wander if the range is set to 0 or 1. They start being able to move at 1.0000000597 follow range (that's as far into the decimals I tested through trial and error, it probably goes much further). Makes it impossible to have something like wandering neutral hostile mobs. I guess you can have completely passive hostile mobs that sit there and listlessly look around though... Come to think of it, wasn't there a time when mobs were able to look at other mobs in the same way they look at the player? Whatever happened to that anyway...? Ah, actually it seems like villagers still have that kind of AI after a quick test, figured they might've still had a feature like that haha.

Also, as I accidentally just found out, animals "see" you from the usual distance when you're holding their food in your hand and will attempt to follow you, but can only ever actually move if you're within their follow range's value.