Deponent verbs are those who are written (normally) in passive form but are active in meaning. loquor, loquī, locūtus sum is a common example in Latin. I wonder if the opposite exists, i.e. a verb which is active in form but passive in meaning. To be honest, I cannot even think of one in Spanish or English, which might indicate that this does not exist in Latin either. Any ideas?
I would argue that iacēre is of this kind. Morphologically it is fully active, but semantically it can be seen as a passive form of iacĕre. Lewis and Short describe it as "to be thrown" and hence "to lie". However, iacēre is not syntactically fully passive: It is intransitive so it takes no objects, but to my knowledge it cannot take an agent.
There may well be more verbs like this, but the pair iacere/iacere is the most common one I know. Many verbs of the second conjugation describe a state rather than an action, which is well compatible with the -ē- working as a passive indicator here.
Another common example that comes to mind is vapulo, -are, which means "to be beaten."
In at least one case cited in L&S from Quintilian's Institutio Oratoria, vapulo can even be paired with an ablative of agent:
. . . testis in reum, rogatus an ab reo fustibus uapulasset, 'innocens', inquit. . . .