Just a note on the Fireforge Games minis - they recently ditched bases for more sprues. So almost all of their kits gained an additional sprue. 12 guys became 18 guys, 6 cav became 9 cav, 24 guys became 30 guys, etc.
Also, while probably not a cheap option, there are some cool minis in some of the 'Epic Encounters' pre-made box sets for D&D. They sell a box of 12-20 plastic minis and slap a map and little pamphlet in there, but they do some unique creatures and that includes some dudes riding stuff. The boxes can occasionally be had for pretty cheap on sale. Actually probably really good little skirmish forces for a lot of games.
Each box of "minions" is linked to a box with the big bad guy (they're separate), but you can see some snake riders here:

Some goblins riding toads here:

Lizardmen on...lions?

Orcs on armoured bears?

Needless to say you only get 2-4 per box, but just a thought.