Scourged Whelpling
Source: Pet Battle. Kel'Thuzad once nursed a plague-infected whelp to adulthood, feeding it only the finest liquefied remains. Undead type. TL;DR - The Daily Respawn Theory has helped several users catch and , and the newest spawn-timer info has been added to this post.
The Scourged Whelpling is a wild pet found in Northrend, requiring patience and precise timing to catch due to its very long spawn timer.
Guide awaiting review
Travel to Sindragosa's Fall
Head to Sindragosa's Fall at 72.95, 31.36 in Icecrown. The Argent Crusader's Tabard is useful for quickly reaching this area.
Log in after server maintenance
Scourged Whelplings have a very long spawn timer, but logging in immediately after server maintenance causes the area around Sindragosa's Fall and Silent Vigil to be filled with spawns, giving you the best shot at finding and battling one.
Skip farming for rare quality
Do not invest time farming for a rare-quality Scourged Whelpling — 
Flawless Battle-Stones can be used to upgrade a lower-quality pet to rare, making quality farming unnecessary.
Tips
Silent Vigil is a secondary location alongside Sindragosa's Fall at 72.95, 31.36 where Scourged Whelplings have been confirmed to spawn.
RPPVP realms and cross-realm zone workarounds have helped players find spawns when their own realm's population is too high.
