A hedgehog died after being spotted engulfed in a "ball of flames" after being set on fire.
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A passer-by tried to save the animal as it was attacked in a park in Aberdare, Rhondda Cynon Taf, between 17:00 and 18:00 BST on Thursday.
They gave first aid but a decision was later taken to put the creature down, according to Hedgehog Helpline.
South Wales Police has appealed for anyone who may have "seen a group of youths acting suspiciously".
Warning: This article contains details and images some may find distressing.
Sarah Liney from the helpline said: "The hedgehog was taken to one of our field hospitals in Tredegar, but the injuries were terrible.
"The animal was sedated but the decision was taken... to put it down."
"When the spikes are burned they continue to burn even when they appear to extinguished.
"The animal must have been in terrible pain. Who would torture an animal like this?"
Hedgehog Helpline said on its Facebook page "youths" were behind the attack which, it said, was "horrific".