Background:
Rattlechains cannot allowBaleful Eidolon to be bestowed in instant speed due to rule 702.102c (or rather 601.e) in conjunction with rule 205.1a.

But Prophet of Kruphix has a special wording. It refers to creature cards, not creatures or creature spells.

Is Baleful Eidolon still a creature card, despite not being a creature when cast with bestow?

If not, will Anafenza, the foremost not exile the aura version of Baleful Eidolon if destroyed by e.g. a Disenchant?

Is there any section in the comp rules that specifies exactly what is meant by “creature card”?

Edited Olle Liljefeldt (May 23, 2016 11:16:15 PM)

There is no functional difference between the wording used on Rattlechains and the one used on Prophet of Kruphix. (Apart from Rattlechains only working on Spirits, of course.) As such, both cards interact the same way with Bestow; if you're casting the card as an Aura, it's not a creature (or a Spirit), and therefore Rattlechains/Prophet doesn't care about it in any way, so you're stuck with the normal timing permissions for that spell.

As for your followup question, there are two rules in the CR that define what is and is not referred to as a card; they are as follows:

108.2. When a rule or text on a card refers to a “card,” it means only a Magic card. Most Magic games use only traditional Magic cards, which measure approximately 2.5 inches (6.3 cm) by 3.5 inches (8.8 cm). Certain formats also use nontraditional Magic cards, oversized cards that may have different backs. Tokens aren’t considered cards—even a card that represents a token isn’t considered a card for rules purposes.

108.2a In the text of spells or abilities, the term “card” is usually used to refer to a card that’s not on the battlefield or on the stack, such as a creature card in a player’s hand. The term “card” can also refer to a card in any zone that’s moving “from anywhere.” On rare occasions, the text of a spell or ability may refer to a nontoken permanent as a “card . . . on the battlefield.” For more information, see section 4, “Zones.”