There are 8 total possible horse types that can be tamed. Two Tier 1 horses (T1A, T1B). Two Tier 2 horses (T2A, T2B). Two Tier 3 horses (T3C, T3F). There are only one each from Tiers 4 and 5 (T4F, T5J).
They can certainly be tricky to distinguish from one another, especially T1B and T3C. The Tier 5 horse is very similar in appearance to others, except it has a black/darker hue.
Thanks to Violet Astray for the following image, showing the different horses that can be caught in the wild and what they look like. For more details on how to differentiate between the horses, check out her site that has closeups of each horse in her article BDO Wild Horse Identification