a rattlesnake

• -557, 969, -52 (seen once)
• -583, -1013, -63 (2 of 4 spawns here)
• 0, 0, 20321 (seen once) (2 of 29 spawns here)
• -941, 1044, -40 (seen once) (1 of 2 spawns here)
• -823, 2693, -52 (seen once)
• 710, 884, -52 (seen once)
• 513, 3629, -52 (seen once)
• -935, 790, -14 (seen once)
• 486, 3686, -52 (seen once)
• -516, 2058, -52 (seen once)
• 974, 2026, -41 (seen once)
• -683, 527, -52 (seen once)
• -523, 246, -47 (seen once)
• -279, 223, -37 (seen once) (1 of 3 spawns here)
• 706, 1770, -37 (seen once) (1 of 3 spawns here)
• -814, 2258, -44 (seen once)
• -570, 3010, -52 (seen once)
• -294, 311, -41 (seen once)
• 403, -556, -41 (seen once)
• -614, -972, -68 (seen once)
• -827, -1004, -29 (seen once)
• -933, 909, -52 (seen once)
• -403, -697, -21 (seen once)
• -630, 2497, -52 (seen once)
• 705, 1494, -52 (seen once)
• -689, 2924, -47 (seen once)
• -232, 1393, -38 (seen once)
• -719, -1537, 2 (seen once)
• 546, -74, -52 (seen once)
• -691, 1012, -52 (seen once) (1 of 2 spawns here)

As /loc gives them: Y, X, Z, the most used first; nearby spawns share a spot, and one seen in a single visit may be where the mob had been pulled to.