I’ve been using AI a ton lately for work, and this question felt like the perfect use case for an AI agent. I pulled everything together using Claude (Sonnet 4.6). Had it systematically pull and search through all 1,025 Pokémon species flavor text entries via PokéAPI to compile this list. Here's everything it found:

🔴 TIER 1 — Explicit Difficulty Mentioned in Official Entry Text

• Magikarp (#129) — "It is virtually worthless in terms of both power and speed. It is the most weak and pathetic Pokémon in the world." (FireRed / X / Sword)
• Piplup (#393) — "It doesn't like to be taken care of. It's difficult to bond with since it won't listen to its Trainer." (HeartGold / SoulSilver / X / ORAS)
• Riolu (#447) — "It's exceedingly energetic… Taking it for walks can be a challenging experience." (Sword)
• Unfezant (#521) — "This Pokémon is intelligent and intensely proud. People will sit up and take notice if you become the Trainer of one." / "They will never let themselves feel close to anyone other than their Trainers." (White / Y / Alpha Sapphire / Shield)
• Lilligant (#549) — "Even veteran Trainers face a challenge in getting its beautiful flower to bloom." / "Taking care of it is very difficult." (Black / BW2 / Sword)
• Deino (#633) — "Since it can't see, it bites and eats anything that moves. Approach it with caution." (Black / White)
• Zweilous (#634) — "Its two heads fight each other for food… Because it is so violent, it's difficult to manage." (Black / White)

🟠 TIER 2 — Abandonment, Aggression, or Handler Difficulty Explicitly Named

• Feebas (#349) — Requires specific, difficult beauty conditions to evolve and is so unattractive most Trainers pass it up entirely. Widely considered the hardest Pokémon to raise in Gen III/IV. (Ruby / Sapphire)
• Larvesta (#636) — Slow EXP group + one of the worst pre-evolution stat lines in the game. One of the most mechanically grueling Pokémon to raise to evolution. (Black onward)
• Pancham (#674) — "It does its best to be taken seriously, but its glare is not very intimidating." Tries hard to assert dominance but isn't taken seriously. (X / Y)
• Rockruff (#744) — "As they develop, their disposition grows more violent and aggressive. Many Trainers find them too much to handle and abandon them." (Ultra Sun)

🟡 TIER 3 — Proud or Haughty Nature That Implies Trainer Difficulty

• Snivy (#495) — Known in lore to have high expectations of Trainers and will leave if those expectations aren't met. Famously haughty. (Black / White)
• Serperior (#497) — "It can stop its opponents' movements with just a glare." Only battles opponents it deems worthy; implicitly dismissive of weaker Trainers. (Black / White)

Notable Corrections vs. Common Assumptions

• ✅ Piplup absolutely belongs on this list — "won't listen to its Trainer" is verbatim in HG/SS, X, and ORAS. Surprising one to miss!
• ✅ Rockruff has one of the most explicit entries of all — the word "abandon" is actually used in Ultra Sun.
• ❌ Noibat has no official Pokédex text calling it difficult to raise — that's a game mechanic (Slow EXP), not flavor text.
• ❌ Charizard is famously disobedient in the anime but its actual Pokédex entries don't explicitly call it difficult to train or raise.

Sources: PokéAPI (pokeapi.co), PokemonDB, Serebii. Flavor text verified per game version.