The chapter is told from Bran's perspective. When he wakes up, Meera tells him it is a pig.
Falling snow and feasting wolves began to dim. Warmth beat against his face, comforting as a mother's kisses. Fire, he thought, smoke. His nose twitched to the smell of roasting meat. And then the forest fell away, and he was back in the longhall again, back in his broken body, staring at a fire. Meera Reed was turning a chunk of raw red flesh above the flames, letting it char and spit. "Just in time," she said. Bran rubbed his eyes with the heel of his hand and wriggled backwards against the wall to sit. "You almost slept through supper. The ranger found a sow."
Behind her, Hodor was tearing eagerly at a chunk of hot charred flesh as blood and grease ran down into his beard. Wisps of smoke rose from between his fingers. "Hodor," he muttered between bites, "hodor, hodor." His sword lay on the earthen floor beside him. Jojen Reed nipped at his own joint with small bites, chewing each chunk of meat a dozen times before swallowing.
The ranger killed a pig. Coldhands stood beside the door, a raven on his arm, both staring at the fire. Reflections from the flames glittered off four black eyes. He does not eat, Bran remembered, and he fears the flames.
A Dance with Dragons, Bran I
But we are given plenty of hints that this might not be the case. First of all, it's north of the wall, a desolate place. Every settlement they find has been deserted. What are the chances of finding a pig, a domesticated pig?
The unlikeliness of this is enhanced by the fact that, as you pointed out, there is readily available meat in the form of dead rangers closeby.
"What happened to the men? The foes behind us?"
"They will not trouble you."
"Who were they? Wildlings?"
Meera turned the meat to cook the other side. Hodor was chewing and swallowing, muttering happily under his breath. Only Jojen seemed aware of what was happening as Coldhands turned his head to stare at Bran. "They were foes."
Men of the Night's Watch. "You killed them. You and the ravens. Their faces were all torn, and their eyes were gone." Coldhands did not deny it. "They were your brothers. I saw. The wolves had ripped their clothes up, but I could still tell. Their cloaks were black. Like your hands."
So all in all, it is very unlikely that it is a pig.
- It's near impossible to find a pig
- There was plenty of other meat to serve as food instead.