An extremely rare herb, one is needed for each flask that an alchemist crafts. Only one black lotus is up at a time, and it can be found in Silithus (one north of AQ gates, one in crystal vale) and Winterspring (one at Mazthoril blue dragon area, and one south of the main road in the western Furblog area). Apparently also one in Eastern Plaguelands, I never saw it.
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Only really to expand on Gráinne's comment as regards the 'collecting' and 'roster development' sections - which are, as said 'distinct' although they are intrinsically linked - I would like to point you in the direction of the in-game Achievements > Battle Pets > Collect tab. There are a series of achievements called Raiding With Leashes, pretty much all of which can be completed via the auction house - if you have the gold/inclination - or by completing old raids.