There are many ways to do random encounter tables. Here’s my favorite: the rolling table.

Setup

Make a list of creatures that are roaming the area and why they’re roaming (looking for food, safety, supplies, intruders in its territory, the key it lost in room 9, etc.). You might sometimes have a second die roll for what it’s up to.

This list might be, say, eleven items long. The first few should be things you want the heroes to encounter early, & the last few should be those you want to appear late. You could have the same creature on your list twice, so that it can appear all thru the adventure.

Use

When you determine that the heroes have encountered something (1 on a 1d6, usually), roll 1d6 on your list. Then cross off item 1, whatever it is; if you didn’t roll a 1, the heroes will never encounter that thing. The next time there’s a random encounter, roll 1d6+1 & cross off item 2. Each time there’s a random encounter, increment the bonus & cross off the lowest item. That way, you’ll slowly work thru the list, removing early encounters & progressing to late encounters.

If you have eleven items, then after five you’ll have six left, so you can stop marking off items each time. At that point, maybe start simply choosing encounters you really wanted to include, whether you’d previously marked them off or not.

Example

1d6+	Encounter
1	Wild boar minding its own business.
2	Evidence of wolves (tracks, dung).
3	Evidence of goblins (tracks).
4	Evidence of owlbear (tracks, dung, remains of a deer it ate).
5	Wolves… (1d6: 1-2=hunting; 3-5=patrolling their territory; 6=fleeing goblins or owlbear).
6	Rootfoot the treant, looking after her forest and searching for Barkbeard (see 11).
7	6 goblins… (1d6: 1-3=foraging; 4-5=patrolling their territory; 6=fleeing the owlbear).
8	Undry the leprechaun, out looking to make mischief.
9	Evidence of owlbear (tracks, dung, remains of a deer it ate).
10	The owlbear out hunting.
11	Barkbeard the treant, looking after his forest and searching for Rootfoot (see 6).

Repeats

Not everything on the list needs to be something the heroes can encounter multiple times. If you use a one-time encounter, just mark it off the list as well (or perhaps instead of the current lowest entry) and, if it gets rolled again, either re-roll or bump it up to the next valid entry.