The Corrupted Shard is a secret legendary pet in BGS it could be obtained from hatching the Corrupted Egg the chances of hatching it are 1 in 4.04M (0.000036%) or 1 in 2.02M (0.000072%) with the Lucky Chances Gamepass.
Mythic is 1 in 808M (0.0000000153%) or 1 in 404M (0.000000036%) with The Lucky Chances Gamepass