As TibiaWiki states you obtain the Phant mount by trading 8000 Drome points with a Tibiadrome NPC. This means you need to participate in the Tibiadrome to collect points. In this related TibiaQA question, What are drome points, how to get them and what can I get for them? You have to complete waves in the Tibiadrome arena. You get 10 points for each wave completed. The points are only awarded after the current rotation is completed which will take 2 weeks.
Here's my Phant Mount-
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