While Ononokusu is very interesting with that huge attack, I am not sure it will actually be better than what we already have.
As a DDer, Salamence has more bulk with Intimidate + better resistances/immunities + better defenses. Ononokusu isn't a glass cannon, but it isn't exactly taking neutral hits that great, especially given the big boost in power seemingly everything received this generation. Salamence can still switch in on physical attacks and ground moves especially to get its dance off. In terms of power, Ononokusu has the edge, but not by that much it has <10% higher attack than Mence, I haven't run calcs or anything but its Jolly attack is less than Mence running Adamant, not exactly game changing it would seem.
Ononokusu does lack a 4x Ice weakness and a weakness to stealth rock (if it still exists), which is a significant advantage, but its frailty evens the playing field in that respect (it will be interesting to see if roost is a tutor, giving Mence another point).
Then of course, there is the speed argument. Mence has that base 100 speed that is perhaps it's biggest advantage over Ononokusu. Ononokusu will have to deal with being outsped by base 100 Scarfers (if scarf still exists) after a DD. Mence can at least tie with them.
As an SDer, unless it gets a powerful priority move, I don't see it having much over Chomp. It lacks dual STAB that Chomp gets, less bulk, no SR or SS resistance. Yes it has more power and yes it has no Ice weakness, but those do not stop it from being outclasses as an SDer by Chomp and his 102 base speed.
Overall I just think that the Ononokusu hype is a bit much. Yes I just compared it to two pokemon that broke the Gen 4 metagame, but this is a completely new ballgame, Ononokusu seems to be getting Rampardos treatment because of that attack stat, I don't think it will suck like Rampardos, but it won't be too powerful either.
(I realize this is all subject to change as it could have access to some base 80 power dragon type priority move or something that will shit over the rest of the metagame that we have not discovered yet, but for now these are my thoughts).