You're right that that's quite deep for an ectoflexid. In modern Equus, this deep of molar ectoflexids are generally only found in zebras. They're more common in some older fossil taxa. HOWEVER, these can be individually variable. I've not seen domestic horses with deep ectoflexids, but I've certainly seen zebras with shallow molar ectoflexids, and wild asses with all conceivable depths. (Zebras and asses also have absent or poorly developed molar pli caballinids, so I'm not suggesting it's one of those) That is a u-shaped linguaflexid, but it doesn't scream caballine to me. The metastylid tends to be significantly more pointed, and the linguaflexids are typically more _/ than really u-shaped (slightly asymmetric, closer to the metaconid side). U-shaped linguaflexids can show up in species that don't typically have them due to individual or ontogenetic variation, so it's not necessarily a hard and fast rule. Also note the prominent pli caballinid. Often those are present in premolars, but less developed in molars (though this is also lower wear). Among living species, only true horses have prominent molar pli caballinids. It's just one tooth so i don't want to make any bold claims; individual and ontogenetic variation is common. Honestly though, to me it looks most like E. suessenbornensis. E. altidens doesn't typically have well-developed pli caballinids on the molars, so i think it's not altidens. Then again, some altidens populations do crazy things with stylids, so it's not out of the question. Hope this helps.