P!nk RIPS Selena Gomez -- Don't PAINT Horses!

5/21/2011 3:30 AM PDT

P!nk RIPS Selena Gomez -- Don't PAINT Horses!

P!nk was SO FURIOUS when she noticed two painted horses on the set of a "stupid music video" for Selena Gomez this week -- she called on animal activists to organize a protest against Justin Bieber's GF.

It all went down Thursday afternoon -- when P!nk noticed a pink horse and a purple horse on a So Cal beach .... and twittered, "If there are any animal activists around Malibu- at Leo Cabrillo State Beach, there are horses being painted for a stupid music video. Shame."

The pop star continued, "Artists should be more aware and responsible for their actions."

Turns out, Gomez was using the pretty ponies for her new single, "I Love You Like A Love Song" -- but her reps are adamant the animals were treated with utmost respect ... at Selena's request.

Selena's rep tells us, "A non toxic, vegetable based powder paint was applied via an airbrush and removed with water. An official from the Humane Society was on set supervising."

Still, we gotta ask ...