Yes, you can change this. It can be done by editing the scenes.txt under the entry of the map you're working on. It's the part right after the file name. See these two maps, for instance, both of which have no horses:
scn_stamford stamford 6400 none none 0.000000 0.000000 200.000000 200.000000 -100.000000 0x00000000300005000003a4e7000073a200002dcf00003c27
0
0
outer_terrain_plain
scn_swanamere swanamere 2304 none none 0.000000 0.000000 200.000000 200.000000 -100.000000 0x00000000400046820005213f00002fea0000174b0000078f
0
0
outer_terrain_snow_hills
I think 6400 means no horses + muddy water, while 2304 means no horses + clear water.
(Thanks, Bassa, for helping me retrieve the codes.)