As I was typing the question, the answer came to my memory.
Ents seemed to be pretty democratic (seemingly verging in anarchist in everyday life) - they had a council (Entmoot) where the decision on whether to go to war was being made. They had respected leaders but apparently no monarch making decisions - even Treebeard had to convince other Ents, despite being the only "elder" left who was active.
'Only three remain of the first Ents that walked in the woods before the Darkness: only myself, Fangorn, and Finglas and Fladrif – to give them their Elvish names...' {{I'm too hasty... so I skipped the rest of the quote with loooooooooong explanation of why he was the only one of the 3 elders dealing with the matter}}
... Still, I daresay I could get together a fair company of our younger folks – if I could make them understand the need: if I could rouse them: we are not a hasty folk. What a pity there are so few of us!'
... 'Hoo, eh? Entmoot?' said Treebeard, turning round. 'It is not a place, it is a gathering of Ents – which does not often happen nowadays. But I have managed to make a fair number promise to come. We shall meet in the place where we have always met: Derndingle Men call it. It is away south from here. We must be there before noon.'
And it was very explicit that they were deciding, all together:
'...and after that we shall have to decide what to do. However, deciding what to do does not take Ents so long as going over all the facts and events that they have to make up their minds about.
(source: Lord of the Rings - The Two Towers, Chapter IV, Treebeard)