Note: based on Table 39 of the 2E AD&D DMG. 
Creatures that do not have character classes and levels calculate THAC0s differently, basing it on the Hit Dice of the creature. When a creature has three or more points added to its Hit Dice, count another die when consulting the table (i.e 8+3 HD = THAC0 11).

Creature Hit Dice       THAC0

1/2                     20
1                       20
2                       19
3                       17
4                       17
5                       15
6                       15
7                       13
8                       13
9                       11
10                      11
11                       9
12                       9
13                       7
14                       7
15                       5
16                       5
17                       3
18                       3
19                       1
20                       1
21                      -1
22                      -1
23                      -3
24                      -3


Note: creature THAC0 values above 16 HD are not present in PnP. They are based on the established progression rate. Also, according to High Level Campaigns, creatures have no THAC0 limits so they could theoretically get negative THAC0 values above 20 HD.
