The low-Level members are named after conservation biology terms, while Level 11 members modify the theme name into "Archnemeses" and are named with the Greek words for "first" and "last". The theme itself might be a reference to Nemesis, an hypothetical star orbiting the Sun at the extreme edge of the Solar System that was proposed as possible cause of Earth's mass extinctions by way of pushing asteroids into the inner system.
The "Nemeses" theme is divided into two categories of monsters: Level 11 "Archnemeses" monsters that focus on destroying and floodgating all monsters of a specific Type or Attribute, and Level 1-4 monsters that focus on shuffling banished monsters back into the Deck to summon themselves.
The Level 11 "Archnemeses" monsters consist of "Archnemeses Eschatos" and "Archnemeses Protos". Both are Special Summon-only monsters that require banishing 3 monsters with different Types ("Eschatos") or Attributes ("Protos") from the player's GY and/or face-up field to be Special Summoned from the hand. Their on-field effects lets them destroy all monsters of a specific Type ("Eschatos") or Attribute ("Protos"), and then locks both players from Special Summoning monsters of that Type/Attribute until the end of the next turn, a devastating effect against Decks that utilize key monsters of a single Type/Attribute.
The Type/Attribute declared to activate the "Archnemeses" monsters' effects must be one that is present on the field; as such, if they're the only monsters on the field, they can only declare Dragon-Type monsters ("Eschatos") or DARK Attribute monsters ("Protos") for their effects. The "Archnemeses" monsters also cannot be destroyed by card effects, and as such are immune to their own destruction effects if their own Type/Attribute is declared for their effects.
The low-Level "Nemeses" monsters each have two effects: the first effect lets them Special Summon themselves from the hand by shuffling 1 banished monster, excluding another copy of themselves, into the Deck. The second effect lets them add a different "Nemeses" monster from the Deck, Graveyard, or Banish Zone to the hand, excluding "Keystone", who adds itself back to the hand if it is in the Banish Zone in the End Phase.
The synergy between low-Level and high-Level "Nemeses" monsters is rather awkward: while low-Level "Nemeses" can theoretically fulfill the summoning condition of the "Archnemeses", as all of them have different Types and Attributes, their need for existing banished monsters to Special Summon themselves means that in a pure build, they can only effectively swarm the field after the "Archnemeses" had already banished 3 monsters and summoned themselves, making this synergy pointless.
Because of their awkward interactions, low-Level and high-Level "Nemeses" monsters are typically played separately as assist cards in non-"Nemeses" decks. The "Archnemeses" are played in decks who can easily fulfill their Type/Attribute and field/GY setup requirements, and/or benefits from having banished cards, to provide extra boss monsters. The low-Level "Nemeses" meanwhile are played in decks that frequently banish cards to provide additional monster swarming and recovery for banished monsters.