Consider section 121a4, which lists a variety of consonant stem adjectives that do not take -i in the ablative singular. They are completely regular, and the entry is really there only to keep you from having second guesses.
I’ll list them here so they get some web mileage, despite not being especially interesting, however rare:
caeles, caelitis relating to the heavens or their Gods
compos, compotis possessing control of
dēses, dēsidis lazy
dīves, dīvitis wealthy
hospes, hospitis amicable, relating to guest-friendship
particeps, participis participating in
praepes, praepitis nimble, winged
pauper, pauperis poor, destitute
prīnceps, prīncipis princely, noble
sōspes, sospitis safe and sound
superstes, superstitis surviving
As someone pointed out in a comment, the general but non-binding idea is that those adjectives which most often operate as nouns (like these) take the ablative in -e, whereas those that are properly adjectival take the ablative in -i. You find hints of this throughout the consonantal and i-stem entries in Allen and Greenough (see 121a1-2), but they make no effort to propagate it as a formal rule.
Yeah, I made that genitive up, but only to describe a real phenomenon in Latin! Some adjectives of likeness, nearness, and belonging that normally take the dative will occasionally take a possessive genitive. This transition is especially common where the adjective approaches the force of a noun.
Fuit hōc quondam proprium populī Rōmānī: this was once peculiar to the Roman people. (~a peculiar trait of)
Fuit semper amīcus Cicerōnis: he was always friendly with Cicero. (~a friend of)
Adeō patris similis es: you’re just like your master. (~a chip off the old block)
Here’s the full list of adjectives that perform this function—
aequālis, aequāle: of the same age (~a contemporary of)
affīnis, affīne: related to by marriage (~kinsman of)
aliēnus, -a, -um: belonging to another (~a stranger to)
cōgnātus, -a, -um: fellow-born (~kinsman of)
commūnis, commūne: common to (~kinsman of)
cōnsanguineus, -a, -um: sharing a bloodline (~kinsman of)
contrārius, -a, -um: opposite (~the opposite of)
dispār: unlike (dispar suī, in philosophical diction)
familiāris, familiāre: of close relation (~intimate of)
fīnitimus, -a, -um: adjoining (~neighbor of)
inimīcus, -a, -um: hostile to (~enemy of)
necessārius, -a, -um: connected with (~component of)
pār: equal to (~a match)
pecūliāris, pecūliāre: personal (~peculiar trait of)
propinquus, -a, -um: neighboring (~neighbor of)
proprius, -a, -um: personal (~peculiar trait of)
sacer, sacra, sacrum: holy (~holy with respect to some deity)
similis, simile: alike to (~spitting image of)
superstes: surviving (~survivor of)
vīcīnus, -a, -um: neighboring (~neighbor of)
Note that this genitive construction is actually more common for proprius, -a, -um than the dative construction.
Similis with the genitive is especially common with personal pronouns (meī, tuī, suī) and within the fixed phrase vērī similis (probable).