Rattata are the only pokemon which are strongest at birth and grow weaker as they gain experience. This is not a strength every Rattata possesses, but one which requires a held item and is passed down through generations. Consequently, there are none who fear wild Rattata, but a trained newborn at the wrong time can terrify even the greatest of trainers.
The strategy relies on the fact that newborn Rattata have not yet developed their decent speed, and the opponent will always attack first, often with a damaging technique. The Rattata, badly wounded but alive because of a Focus Sash, retaliates with Endeavor and Quick Attack. There are countless workarounds which turn this narrow defeat into a narrow victory – poison, ghost-type pokemon. Trick Room, entry hazards, Sandstorm, or even Quick Attack from a faster pokemon, but the need to counter it just speaks to the fact that a newborn Rattata is respected at the top levels of the Pokemon League, and all the counters save ghosts still leave an enemy pokemon badly wounded. It is this strategy which gives rise to stories of Donphan fearing Rattata, and some say the strategy could even defeat Arceus himself.
It is, however, a tactic which is deadly only once in a pokemon's lifetime. Defeating a pokemon so much stronger than itself gives the Rattata a ton of experience, typically forcing them to evolve after the battle. When they evolve they gain the toughness to shrug off a weak attack and the speed to go first in battle, while the strategy relies on going second and being nearly knocked out. Some trainers continue to train them as Raticate, but it is rare for these trainers to make it very far at the Pokemon League.