Both spearmen and ulfsarks can get upgraded anti-cav bonuses.
The spearman has pretty uninspiring performance against the better cavalry without extensive myth upgrades, but by the time you get camels it may be a non-issue.
Elephants are nasty indeed, and steamrole most things on a population basis, but really what they've got going for them is extreme population efficiency. They have more hitpoints and hack armour than most cavalry, so they can actually out last infantry in mele. You have to match them on basis of cost to do very well against them, save you have katapeltes which eat elephants for lunch.
If an Odin player worships both Freja and Njord for their respective Jarl/cavalry bonuses, the result is (to me) underwhelming. It does sit on a stack of hitpoints (3oo and some odd), but is only a little faster than an ulfsark. The attack is also lower than several late-game infantry units. They do have the advantage of moving at exactly the same rate as the swifter Odin ulfsarks, so a few can be mixed in to make the mix more resistant to infantry and other cavalry.
Posiedon hippikons are relatively easy to mass, but are straightforward to kill. Counter cavalry such as podromos and camels, or just about anything that deals good hack and has good hack armour itself. I've never seen massed hetaroi, somehow I think it wouldn't work.
I'm curious, how well does the contarius hero w/ theia upgrades compare to the jarl? I know that they're not anywhere as tough, but they're faster and get an anti-building bonus. IIRC the anti-myth bonus is comperable as well.