London Zoo says it believes Melati carried the cub outside, but keepers are unclear as to how the cub got into the pool.
Keepers are now reviewing the situation because it was not forseen that the mother would take the cub outside so early.
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The impact of the 4mph crash pushed the buffers back by over a metre.
The 18-month-old puppy lives with owners Chris & Gillian French and their 10-year-old daughter Mia at the Mitre pub in Richmond-upon-Thames.
Detectives are hunting two suspects after the 23-year-old victim was left with a broken rib and a deep cut to his lung.