Do We Know What Kind of Creature Malack Is?

[Deepbluediver]

Even though he said he was lizardfolk?

Durkon is the one who first calls Malak a lizard, and while Malak does refer to "us lizardfolk", I don't believe it's outside the realm of possibility that this is not exactly correct.
For example, I'm pretty sure the MM has an entry of half-yaun-ti who have snake-features but also legs and feet.

I admit it seems unlikely, but I think it's possible that perhaps Malak either didn't know he's yaun-ti (which might explain some of the health problems) or he does know, and is keeping it a secret. I recall that the fluff in the MM describes the Yaun-ti as being one of those hate-everyone races, who everyone is happy to hate right back.

Again, highly unlikely, but plausible IMO.