Is there any point in keeping more than one Entei if one has FS/OH and much better IVs than the rest? I have seven and transferring six would free up storage space and save Rare Candy for powering up my best one. Ho-Oh and Groudon have multiple useful movesets (an obvious reason to keep multiple), but not Entei.
The weather makes catching wonders much easier. What can I do with multiple wonders of the same species (like Ponyta) whose evolved form is not and will never be meta-relevant, besides using them as evolution fodder for experience? How about for species that cannot be evolved (like Dunsparce) and will never be meta-relevant?
I dont think so. Sadly Entei is stuck in the middle. Moltres has the edge in attack and Ho-oh has much more defense and overall stat. He is a cool Pokémon but it is unlikely that he ever gets relevant.
For the other ones you gave the response yourself. They are not relevant. Many trainers keep the best IV one but personally I dont see any sense in that.
Entei is still a fine Pokemon and not that much worse than Moltres. It's also better than Ho-Oh, because Ho-Oh is not much of a Fire attacker with its moveset. The problem isn't that Entei is bad so much as the meta disfavors Fire types right now.
Regardless, I wouldn't get rid of ALL your Enteis, but getting rid of the chaff is fine. I'm personally saving my extra legendaries for the next double candy event, but if you don't have the storage space and don't want to shell out 200 coins for more, then getting rid of them now is reasonable.
I wouldn't. Six slots on an unobtainable Pokemon won't go very far, especially if you missed out on Moltres. Entei is a good rounded Fire-type that fills a niche that Ho-Oh and Groudon don't as a pure Fire attacker.
A lot of people like Entei over Moltres because it can learn FS-FT, giving it a two-bar option to sweep gyms or as a style preference. You may want to consider that as an option (I personally don't see anything wrong with using a Flareon for that instead).
Either way, it's only 6 slots.
The only reason I can think of to keep more than one Entei is if you don't have enough powerful fire-types to throw six of them at a raid that is weak to fire, such as Scyther.
However, if you also have Moltres and a few good Flareon, you probably don't need the Entei. If you don't have six powerful fire-types, consider keeping some Entei around.
Fire is going to be very relevant for gen3. Look at the raid bosses. At least 4 future legendaries, fire types like moltres and entei will be king. Moltres has highest dps while entei has highest tdo. I would keep a few extra. Level 20 still good in clear weather. I personally maxed out 2 entei to 40.
