In most cases, AD&D monsters exist in 3e/3.5e/PF1. There are many sources, between the 5 MM’s, Fiend Folio, and the Tomes of Horrors. Just do a Bing or Google search too - a fan may have converted it.
if you can’t find it, you probably could convert by using the same HD and keeping pretty much the same attacks. AC, take 10 - AD&D AC, add the absolute value to 10 = rough 3/3.5/PF1 AC. The game didn’t completely change until 4e.