Flametongue Yearling is instantly nostalgic for anyone who remembers casting Flametongue Kavu.
Modern Horizons 2 does not utilize subtlety in the slightest, with all of its "modernization" of cards being obvious and evoking similar artwork. Flametongue Kavu is an old favorite that sadly has been lost to the years of power creep (and legality, of course). Flametongue Yearling is the old kavu's stronger offspring.
At 4 mana, Flametongue Yearling is actually slightly weaker, as it is just the same card with 1 less power (and thus 1 less damage). But the most common way Flametongue Yearling will be cast is as a 2 mana 2/1 that kills an opposing creature. This flexibility is a obviously a humongous upgrade to the card. It's just a small added bonus that if a game has gone long, you can sometimes just multi-kick it for 10 mana and have a 6/5 that deals 6 or whatever.
It's worth noting that just like its predecessor, the kavu's fiery ability is mandatory – so you cannot cast it onto an empty board without killing itself. We're OK with that design choice, making the card slightly weaker to preserve the original feel of the card!