I use Grusilda, Monster Masher to mash Rune Tail, Kitsune Ascendant[/card] with any other creature. What does it become when I flip it? A legendary enchantment creature?
With the understanding that no Magic card (other than "silver-bordered" cards) has an effect like that of Grusilda's abilities:
The comprehensive rules don't include any rules on "combined" permanents or what happens when a permanent enters the battlefield "combined". The answer ultimately depends on what rules the players in the game adopt for "combined" permanents (perhaps with inspiration from the reminder text on Grusilda). Flipped is a status under C.R. 110.5, and C.R. 709.2 says that "(o)nce a permanent is flipped, its normal name, text box, type line, power, and toughness don't apply", but rather their alternative versions. See also the example in C.R. 110.5c, which clarifies what happens when a permanent that isn't normally a flip card is flipped.
In general, in a game allowing "silver-bordered" cards such as those in Unglued, Unhinged, and Unstable, the players in the game can agree on modifications to the comprehensive rules ("house rules") to accommodate situations, such as this one, that the comprehensive rules neither regulate nor answer and that are unique to such cards (see also C.R. 100.7). Although Mark Rosewater issues "rulings" on how certain game situations unique to such cards play out, such advice is no more or less valid than the "house rules" agreed to by the players (that is, such players can agree whether to adopt such "rulings" or not).
EDIT (Jan. 16): Correctness edit in view of changes to the Magic Tournament Rules with Rivals of Ixalan.
EDIT (Mar. 4): Edit last sentence of second paragraph; apparently the rules do clarify the situation with permanents not normally flip cards.
EDIT (Jan. 2, 2019): Edited.
EDIT (Jul. 9, 2019): Some rules were renumbered with Core Set 2020.
The comprehensive rules don't include any rules on "combined" permanents or what happens when a permanent enters the battlefield "combined". The answer ultimately depends on what rules the players in the game adopt for "combined" permanents (perhaps with inspiration from the reminder text on Grusilda). Flipped is a status under C.R. 110.5, and C.R. 709.2 says that "(o)nce a permanent is flipped, its normal name, text box, type line, power, and toughness don't apply", but rather their alternative versions. See also the example in C.R. 110.5c, which clarifies what happens when a permanent that isn't normally a flip card is flipped.
In general, in a game allowing "silver-bordered" cards such as those in Unglued, Unhinged, and Unstable, the players in the game can agree on modifications to the comprehensive rules ("house rules") to accommodate situations, such as this one, that the comprehensive rules neither regulate nor answer and that are unique to such cards (see also C.R. 100.7). Although Mark Rosewater issues "rulings" on how certain game situations unique to such cards play out, such advice is no more or less valid than the "house rules" agreed to by the players (that is, such players can agree whether to adopt such "rulings" or not).
EDIT (Jan. 16): Correctness edit in view of changes to the Magic Tournament Rules with Rivals of Ixalan.
EDIT (Mar. 4): Edit last sentence of second paragraph; apparently the rules do clarify the situation with permanents not normally flip cards.
EDIT (Jan. 2, 2019): Edited.
EDIT (Jul. 9, 2019): Some rules were renumbered with Core Set 2020.