Well, let's get right into it. It is obviously shooting fire out of its mouth and hitting another creature. So it at least needs to do damage to creatures. It doesn't have to be upon entering the battlefield. And it doesn't have to be limited to creatures (hit players). You could have an ability like Prodigal Pyromancer's. In fact, you might argue that having a repeatable tap ability like that makes more sense for the art of this card because of this: the flying creature is being hit by one of the kavu's attacks. ...and the kavu is revving up ANOTHER one in its mouth!
Well, that's what's technically happening, but the impression that the viewer has is that the shot is doing the action of traveling from its mouth to the creature in the air as the viewer's eyes move from the mouth to the creature.
...Actually, I've never taken any art classes and don't do art or know art principles or theory, so this could just be all wrong. But whatever - this matters little to not at all.
Hey, how big are kavus? *research research research*
O.K., so they're generally at least 2/2s, but there are a still a decent number of Kavus that are smaller size than this. And a lot of them don't even look smaller than the average Kavu. So, we won't have problem making this Kavu smaller, even though it looks pretty impressive in the art.
Hmm... My next step - I can either next look at what this card should cost or determine what kind of damage ability it should have. I'll go with the latter.
"T: CARDNAME deals N damage to target creature (or player)."
"Whenever another creature enters the battlefield, you may have CARDNAME deal N damage to that creature."
"At the beginning of (your upkeep/combat/), you may have CARDNAME deal N damage to target creature (or player)."
"MANA, T: CARDNAME deals N damage to target creature with flying."
"Whenever you cast a (red) spell, CARDNAME deals N damage to target creature (or player)."
"MANA: CARDNAME deals N damage to target creature."
I'll stop for now.
I don't feel comfortable having a red creature deal damage to creatures with flying - that's a green thing. Red likes to hit things without flying, like when an Earthquake happens. Firespout shows this whole dealio.
But upon thinking about the art more, I see that the drake is getting hit and another piece of fire is getting ready to happen. This means that an activated ability where tapping is a cost shouldn't be chosen. Because it takes too long for the kavu to recover for the art to make sense.
I think it makes for a more compelling creature if it just dealt damage to creatures. That's what the art depicts, anyway - it hurting a creature. I looked for similar creatures for this kind of ability and found Flamekin Spitfire. Which actually has a name that kind of aptly describes what Flametongue Kavu is doing. Except it deals damage to players as well. And that's just fine. The Kavu hurts creatures. ...O.K., maybe we'll let it damage players, too.
Hmm... There we have it! A larger body version of Flamekin Spitfire!
I found that "Flametongue Kavu" as a name is already perfect for what this new ability does, so no need to change it.