You need to defeat the following fel-corrupted pets in Tanaan Jungle:
Felsworn Sentry
Chaos Pup
Felfly
Direflame
Dark Gazer
Vile Blood of Draenor
Netherfist
Defiled Earth
Corrupted Thundertail
Cursed Spirit
Tainted Maulclaw
Mirecroak
Bleakclaw
Dreadwalker
Skrillix
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You'll get guide for completing Tiny Terrors in Tanaan