what does the “rumlin” mean？
“I win,” a long-armed sloth exclaimed.
She collected the pile of loot that had been anted in. The others appeared downright livid. A rumlin seemed a particularly sore loser, gnashing its teeth angrily.
I know it's a made-up word.

Question

what does the “rumlin” mean？
“I win,” a long-armed sloth exclaimed.
	She collected the pile of loot that had been anted in. The others appeared downright livid. A rumlin seemed a particularly sore loser, gnashing its teeth angrily. 
I know it's a made-up word.

Answer

Somewhere in that book, it should explain what a rumlin is. It may be implied. From what I've read in your comments, it could be that "rumlin" is a derogatory term for an elvin pirate. As pirates are known for their love of "rum", it does make a little sense like that.

However, if the writer doesn't make it clear what the word means, that's their failing, not yours.

Answer

I've never read that book, but based on the context, it seems to be some sort of animal.

Answer

“Look,” Skylar said quietly to Aldwyn and Gilbert. “A pair of elvin pirates.” 
the next sentence
rumlin=elvin pirate？

Answer

“familiars”, a fantasy novel

Answer

What is this from?