Deer
The Deer is a 
Tier 4 Pet, available in the 
Turtle Pack. When the Deer faints, it summons a 
Bus with 
Chili. Levelling up the Deer increases the level and stats of the summoned Bus.
Strategy
Deer is one of the pets with the lowest base stats possible in the game. It is also one of the strongest summoner in the game that wants to be knocked out ASAP to get to Bus. An early Bus can sweep plenty of low tier pets with its good stats and Chili, which is a tier 5 food perk. Deer combines well with summon pets such as 
Whale, 
Turkey, 
Tiger and so on.
Pilling a Deer will permanently put Bus on the team, which can work as a good temporary pet due to Chili's relative power in the early rounds, especially if the Deer was found from a level up reward on turn 5.
Trivia
Gallery
The standard sprite of the Deer
The classic sprite of the Deer
The "DeerBus" emoji from the Official Super Auto Pets Discord Server
The "DeerThisIsFine" emoji from the Official Super Auto Pets Discord Server
