you have to find its ancient egg and hatch it to spawn the Sniffer. Unlike the original teaser, the Sniffer egg will spawn inside suspicious sand, which is a new block coming with Minecraft 1.20 alongside archaeology features.
Use an egg timer or something similar--but don't let him see you doing this. At first, you want to make him hold his breath for no more than about 10 seconds at a time. Every day or two, increase his time between breaths by about 5 seconds. Eventually, you'll want him to be ...