Gen 3 Whismur is outstandingly terrible and definitely doesn't work for this category. Keep in mind that Gen 3 Uproar is 50 BP, and that Loudred still isn't particularly strong. Due to the higher power of Headbutt vs Uproar, Zigzagoon is actually a significantly better early game mon for Normal type damage - even on evolution Linoone outpaces Loudred until the Strength HM because Headbutt still has more BP than Uproar or the level 25 Stomp. Even after getting Strength, Loudred is exactly 1 base attack stronger than Linoone, not exactly particularly amazing.
Loudred (and Exploud) actually start coming into their own at the start of lategame, when Loudred finally evolves at level 40, unless time is spent much earlier grinding for Game Corner TMs, since it's actually rather potent with its useful TM movepool - mostly Ice Beam and Flamethrower. The issue for Loudred is that by the time the free Ice Beam comes around Loudred has fallen off pretty well in terms of stats. It's slow, it's not particularly bulky, it's not very strong. It's only when Loudred becomes Exploud and improves its stats that it can actually make use of its good TM movepool. I mean Whismur in general is just kind of terrible in RSE, what with its awful matchup against all three of the early gyms.
Gen 6 Whismur, from what I remember, isn't actually all that much better, since it's got to wait until it's well into the Loudred stage for the 90 BP Uproar, but that at least makes Loudred itself alright. Access to Surf is a nice addition to Exploud though Loudred doesn't get it. From what I remember of the I think one time I used Whismur (I've played a lot more RSE than ORAS), it was still at its best in the early lategame Exploud stage - although it definitely got a big jump at Boomburst right before the league.
If I had to pick a RSE mon to fit this category I'd probably go with Gyarados, but maybe Torkoal or Seviper.
Gyarados fits the "early availability -> can be fully evolved early (with investment) -> massively powerful (and cool!), but will be obsolete later on" rather well in RSE, although obviously Magikarp is somewhat worse than Nidoran. Gyarados itself is incredibly strong with Strength after gym 3, but does kind of fall off lategame due to its lack of physical STAB moves. Earthquake can help keep it somewhat more relevant than RBY attacker nido, but obviously Horn Drill lets Nidoking participate lategame too.
Torkoal actually has very solid stats for when it joins, although it (and Seviper) are really stretching the definition of early availability. A very quick Body Slam and Overheat not too long after turn it into a fairly potent attacker, only aided by Flamethrower not too far off. It does fall off very hard lategame, with terrible matchups against almost everything after the 6th gyms, which is worse than the nidos.
Seviper is pretty much the same boat as Torkoal, only it's stronger and not as slow in exchange for worse bulk. I think that aside from the significantly worse availability of Seviper, it's actually a decent comparison to the nidos, as it has a surprisingly wide TM movepool, is competent both physically and specially, and has largely ok matchups for most of the game, with some of the later stuff being pretty poor overall.
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