From RWP Wiki
The Rareware Cow is an interesting visual easter egg which has been spotted in many of Rare's handheld titles. Similar in fashion to the slices of cheese which can be spotted in Perfect Dark, or the rats which are hidden throughout Perfect Dark Zero - these cows are hidden within the levels of such titles as Banjo-Kazooie: Grunty's Revenge and Perfect Dark GBC.
The origins of the cow are unknown, but it's most likely just a joke among the handheld team's artists.
The cow has currently only been sighted in four of Rareware's Game Boy titles, however it is entirely possible that they're to be seen in Sabre Wulf, It's Mr. Pants, Mickey's Speedway USA GBC, and the remakes of the Donkey Kong Country trilogy.
Sighting 1: Conker's Pocket Tales
He's there - or at least 2 of them, found in the field near the graveyard and the undertakers house in VultureVille.
Sighting 2: Perfect Dark GBC
The cow can be seen graffitied onto one of the walls in the outside of the game's opening level.
Sighting 3: Donkey Kong Country GBC
The cow can be found in the Slipslide Ride level of the GBC remake of DKC - inside one of the bonus barrels.
Sighting 4: Banjo-Kazooie: Grunty's Revenge
The cow can quite clearly be seen in the Bilge Haven area of the Spiller's Harbour level in Banjo-Kazooie: Grunty's Revenge. On entering the level, head west of the docks, towards the Bilge Haven beach. To the left of the gigantic sandcastle - an image of the cow can be seen drawn out in sand. The stick which was used to draw the picture can be seen by its side.